I Have seen several posts regarding this feature to preview HTML file on GitHub. Some suggest prepending https://htmlpreview.github.io/ before the real URL and some suggestions to create GitHub pages that I don't want to do now because then I have to re-create most of the thing on .yml
But my confusion is why this link showing HTML preview but not mine when I prepend the same string on my repo's HTML page. Please let me know If any other way I can see the HTML preview on github
Working:
https://htmlpreview.github.io/?https://github.com/bartaz/impress.js/blob/master/index.html
Not Working:
https://htmlpreview.github.io/?https://github.com/sany2k8/setuptools/blob/master/How_to_Install_Spark_on_Ubuntu.html
Showing 404 Not Found
Error: Cannot load
https://raw.githubusercontent.com/sany2k8/setuptools/master/How_to_Create_Github_Pages.html:
404 Not Found
Check these steps:
-Your repository is public.
(Settings>General>Danger Zone>Change repository visibility>Make public)
-The size of the files are not very large.
-The Repository link is in HTTPS and not in SSH.
EDIT:
I found this question about the private repository and preview, if you want to take a look.
When your html file is excessively big then often github may fail to open the file. In such cases you can check the raw version of the HTML file.
Folks, help is needed and MUCH appreciated with a fuzzy behaviour of Jekyll site deployed to Github.
The site works perfectly when putting it up and using it in the local machine (through 'jekyll serve'). The surprise came up upon deployment to Github, the HTML 'posts.html' page created to show the post list does not get rendered at all. The rest of the site is up and running fine, but once the link is clicked to reach the mentioned page the browser is trying to DOWNLOAD the file, instead of rendering.
If you need to have a peek at the code:
https://github.com/zekdeluca/zekdeluca.github.io
And the site can be seen at:
http://zekdeluca.github.io/
Thanks in advance!
It has to do with the permalink and how the extension-less urls are working. If you add a trailing slash to your url it will work. If you are trying to do it without the trailing slash, I was under the impression that what you did would work on GH.
Something like permalink: /my-page/ will work and it creates a folder called my-page with an index.html file in it. The url will show as /my-page/ without the index.html - pretty except the trailing slash.
If you do permalink: /my-page it should be making a file my-page.html, but in your case it seemed to have made just my-page with no extension, which is then being served as a file by github and it doesn't seem to know what it is so it is downloading it instead of serving it.
There seems to be a lot of conflicting info on this, I think the behavior has changed recently, and maybe the version GH is using is able to output a file with no extension? I did not think that was the case.
more reading:
http://overengineer.net/pretty-extensionless-urls-in-github-pages-using-jekyll/
Jekyll extension-less permalinks with markdown
Since the GitHub wiki does not support directories, I want to put my md files into GitHub pages. However when I open them I found they are not interpreted at all. GitHub just gives me the raw file. See http://greenlaw110.github.com/Rythm/en/index.md. Any idea?
The documentation isn't particularly clear on this, but given that the Markdown rendering is done by Jekyll, I believe you need what they call "YAML front matter" for it to compile the page. So try putting this at the top of your file:
---
title: Document Center
---
That should be enough to trigger Jekyll. (And then you can get fancy and start using layouts, etc.)
Another reason (found by experience) why you might get a raw index.md file, is if you happen to also have a similarly-named index.html file. This blocks GH Pages.
Deleting index.html produces (on the site) a new index.html with parsed markdown. The original index.md is then no longer accessible.
Is it possible to place a screenshot in README file in a GitHub repository? What's the syntax?
If you use Markdown (README.md):
Provided that you have the image in your repo, you can use a relative URL:
If you need to embed an image that's hosted elsewhere, you can use a full URL
GitHub recommend that you use relative links with the ?raw=true parameter to ensure forked repos point correctly.
The raw=true parameter is there in order to ensure the image you link to, will be rendered as is. That means that only the image will be linked to, not the whole GitHub interface for that respective file. See this comment for more details.
Check out an example: https://raw.github.com/altercation/solarized/master/README.md
If you use SVGs then you'll need to set the sanitize attribute to true as well: ?raw=true&sanitize=true. (Thanks #EliSherer)
Also, the documentation on relative links in README files: https://help.github.com/articles/relative-links-in-readmes
And of course the markdown docs: http://daringfireball.net/projects/markdown/syntax
Additionally, if you create a new branch screenshots to store the images you can avoid them being in the master working tree
You can then embed them using:
Even though there is already an accepted answer I would like to add another way to upload images to readme on GitHub.
You need to create issue in your repo
Drag and drop in the comment area your image
After link for the image is generated insert it to your readme
More details you can find here
I found that the path to the image in my repo did not suffice, I had to link to the image on the raw.github.com subdomain.
URL format https://raw.github.com/{USERNAME}/{REPOSITORY}/{BRANCH}/{PATH}
Markdown example 
JUNE 3, 2020 : WORKING ANSWER-
Upload your image to postimage.org
Copy Github-Markdown-Url after uploading Image
Insert in your ReadMe
One line below should be what you looking for
if your file is in repository
if your file is in other external url
The markdown syntax for displaying images is indeed:
BUT: How to provide the url ?
You probably do not want to clutter your repo with screenshots, they have nothing to do with code
you might not want either to deal with the hassle of making your image available on the web... (upload it to a server... ).
So... you can use this awesome trick to make github host your image file. TDLR:
create an issue on the issue list of your repo
drag and drop your screenshot on this issue
copy the markdown code that github has just created for you to display your image
paste it on your readme (or wherever you want)
http://solutionoptimist.com/2013/12/28/awesome-github-tricks/
Much simpler than adding URL
Just upload an image to the same repository,
like:
add this to README
<div align="center">
<img src="/screenshots/screen1.jpg" width="400px"</img>
</div>
From March 2021, this is now supported:
Attaching files to markdown files
You can now attach files, including images, to markdown files while you're editing them in the web.
This works just like file attachments in issues and pull requests and supports the same file types.
Just drag and drag, click and select, or paste.
Note: If you add an image to a markdown file, anyone can view the anonymized image URL without authentication, even if the markdown file is in a private repository.
To keep images private, serve them from a private network or server that requires authentication.
For more information on anonymized URLs see "About anonymized image URLs".
Markdown: 
Create an issue regarding adding images
Add the image by drag and drop or by file chooser
Then copy image source
Now add  to your README.md file
Done!
Alternatively you can use some image hosting site like imgur and get it's url and add it in your README.md file or you can use some static file hosting too.
Sample issue
Method 1->Markdown way
Method 2->HTML way
<img src="https://link(format same as above)" width="100" height="100"/>
or
<img src="https://link" style=" width:100px ; height:100px " />
Note-> If you don't want to style your image i.e resize remove the style part
For me, the relative path worked out very well.
How I did this:
In my current md file where I wanted to use a picture from another directory, I used a relative path like this - consider following points.
md file loc: base dir -> _post -> current_file.md
& picture file loc which I wanted to use: base dir -> _asset -> picture_to_use.jpeg
What the statement I used was on current_file.md file:
Note - before this, I was using direct _asset but Ideally, it so starts from ../_asset/and-so-no
reference - https://docs.github.com/en/github/creating-cloning-and-archiving-repositories/creating-a-repository-on-github/about-readmes
Add  in the readme markdown as mentioned by many above. Replace screenshot.png with the name of the image you uploaded in your repository.
But here is a newbie tip when you upload the image (as I made this mistake myself):
ensure that your image name does not contain spaces. My original image was saved as "Screenshot day month year id.png". If you don't change the name to something like contentofimage.png, it won't appear as an image in your readme file.
I googled a few similar questions and did not see any answers with my problem and its quite simple/easy solution.
Google Cloud Storage - a slightly different approach to images in READMEs
Here goes: like the OP, I wanted an image in my Github README, and, knowing the Markdown syntax for doing so, typed it in:
 for this to work.
But, wait...failure--there's no actual rendered photo! And the link is exactly as given by Google Storage!
Github camo - Anonymous Images
Github hosts your images anonymously, yay! However, this presents an issue for Google storage assets. You need to get the generated url from your Google Cloud Console.
I'm sure there's a smoother way, however, simply visit your given URL endpoint and copy the long URL. Details:
Instructions
Visit your storage console: https://console.cloud.google.com/storage/browser/${MY_BUCKET}?project=${MY_PROJECT}
Click on the image you'd like to display in Github (this brings up the "Object Details" page)
Copy pasta that URL (the one starting with https not gs) into a new browser tab/window
Copy pasta the new generated URL -- it should be longer -- from your new browser tab/window into your Github README file
Hopefully this helps speed up and clarify this issue for anyone else.
Please fallow this four step,
this worked for me
1-Create a new issue on GitHub.
2-Drag-and-drop an image onto the comment field or upload a photo.
3-Wait for the upload process to finish.
4-Copy the URL and use it in your Markdown files on GitHub (use that generated URL in your repository's readme).
First, create a directory(folder) in the root of your local repo that will contain the screenshots you want added. Let’s call the name of this directory screenshots. Place the images (JPEG, PNG, GIF,` etc) you want to add into this directory.
Android Studio Workspace Screenshot
Secondly, you need to add a link to each image into your README. So, if I have images named 1_ArtistsActivity.png and 2_AlbumsActivity.png in my screenshots directory, I will add their links like so:
<img src="screenshots/1_ArtistsActivity.png" height="400" alt="Screenshot"/> <img src=“screenshots/2_AlbumsActivity.png" height="400" alt="Screenshot"/>
If you want each screenshot on a separate line, write their links on separate lines. However, it’s better if you write all the links in one line, separated by space only. It might actually not look too good but by doing so GitHub automatically arranges them for you.
Finally, commit your changes and push it!
To me the best way is -
Create an new issue with that repository on github and then upload the file in gif format.To convert video files into gif format you can use this website http://www.online-convert.com/
Submit the newly created issue.
Copy the address of the uploaded file
Finally in your README file put 
Hope this will help .
Add image in repository from upload file option then in README file
![Alt text]("enter image url of repositoryhere")
Create a New issue by clicking on the green button in the upper right corner. Take a screenshot of whatever you need and paste it into the issue message (CMD+V on Mac or CTR+V on Windows).
My image had space in the name, e.g. Pasted image 20221006123035.png, and that was causing a problem from me, and I lost some time fixing it. If someone has the same issue, then here are the steps
I uploaded the image via upload option
Then clicked on the image in the file browsing UI of the website.
The image is opened in the browser. From the browser's URL I copied the url of the image. You can copy the whole path, or relative to your .md file
Now insert that link to the .md file like 
With the images located in /screen-shots directory. The outer <div> allows the images to be positioned. Padding is achieved using <img width="desired-padding" height="0">.
<div align="center">
<img width="45%" src="screen-shots/about.PNG" alt="About screen" title="About screen"</img>
<img height="0" width="8px">
<img width="45%" src="screen-shots/list.PNG" alt="List screen" title="List screen"></img>
</div>
The root of the site http://example.com correctly identifies index.html and renders it. In a similar manner, I want, http://example.com/foo to fetch foo.html present in the root of the directory. The site that uses this functionality is www.zachholman.com. I've seen his code in Github. But still I'm not able to find how it is done. Please help.
This feature is actually available in Jekyll. Just add the following line to your _config.yml:
permalink: pretty
This will enable links to posts and pages without .html extension, e.g.
/about/ instead of /about.html
/YYYY/MM/DD/my-first-post/ instead of YYYY-MM-DD-my-first-post.html
However, you lose the ability to customize permalinks... and the trailing slash is pretty ugly.
Edit: The trailing slash seems to be there by design
It's actually the server that needs adjusting, not jekyll. Be default, jekyll is going to produces files with .html extensions. There may be a way around that, but it's unlikely that you really want to do go that route. Instead, you need to let your web server know that you want those files served when a URL is called with the file's basename (and no extension).
If your site is served via an Apache web server you can enable the "MultiViews" option. In most cases, you can do that be creating an .htaccess file at your site root with the following line:
Options +MultiViews
With this option enabled, when Apache receives a request for:
http://example.com/foo
It will serve the file:
/foo.html
Note that the Apache server must be setup to allow the option to be set in the htaccess file. If not, you would need to do it in the Apache config file itself. If your site is hosted on another web server, you'll need to look for an equivalent setting.