DD_belatedPNG not working in dynamic HTML node - png

Question

everyone.
The enviroment is IE6.
There are 9 li node has png images, and random display 4 node, html code like this.
<ul>
<li><img src="xxxxx.png"/></li>
<li><img src="xxxxx.png"/></li>
<li><img src="xxxxx.png"/></li>
<li><img src="xxxxx.png"/></li>
<li style="display:none;"><img src="xxxxx.png"/></li>
<li style="display:none;"><img src="xxxxx.png"/></li>
<li style="display:none;"><img src="xxxxx.png"/></li>
<li style="display:none;"><img src="xxxxx.png"/></li>
<li style="display:none;"><img src="xxxxx.png"/></li>
</ul>
I use DD_belatedPNG to display .png images, code like this:
<!--[if IE 6]>
<script type="text/javascript" src="/static/js/DD_belatedPNG.js"></script>
<script type="text/javascript">
try{ document.execCommand('BackgroundImageCache',false,true); }catch(e){ }
DD_belatedPNG.fix('*');
</script>
<![endif]-->
When the page onload, the .png image can display.
But when I use js to display the rest of others png images, something goes wrong. The png images can not be display.
How can I solve this problem. Thanks.