How to stop NSInvocationOperation and NSOperationQueue - iphone

Question

I am using [NSOperationQueue mainQueue] and NSInvocationOperation in table view cellforRowAtIndexPath method to load images from web in background.But it is not stopping after download image. Here is my code
- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath {
UITableViewCell *cell=[mytable dequeueReusableCellWithIdentifier:#"cell"];
if (cell == nil)
{
cell = [[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:#"cell"];
}
imageLabel=(UIImageView *)[cell viewWithTag:5];
UIImage *image=[dict objectForKey:[appSmallLogos objectAtIndex:indexPath.row]];
if (image) {
imageLabel.image=image;
} else {
NSURL *on=[appSmallLogos objectAtIndex:indexPath.row];
NSString *on1=[appNames objectAtIndex:indexPath.row];
NSMutableArray *array=[[NSMutableArray alloc] initWithObjects:on,on1,indexPath, nil];
operation = [[NSInvocationOperation alloc] initWithTarget:self selector:#selector(didGetAlbums:) object:arr];
q =[NSOperationQueue mainQueue];
[q addOperation:operation];
imageLabel.image=[dict objectForKey:on1];
}
return cell;
}
//didgetalbums method
- (void )didGetAlbums: (NSMutableArray* )url
{
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0ul);
dispatch_async(queue, ^{
NSData *data=[NSData dataWithContentsOfURL:[url objectAtIndex:0]];
UIImage *image1 = [UIImage imageWithData:data];
dispatch_sync(dispatch_get_main_queue(), ^{
[dict setObject:image1 forKey:[url objectAtIndex:1]];
});
});
//reload the requested cell after download image
[self. mytable beginUpdates];
[self. mytable reloadRowsAtIndexPaths:[NSArray arrayWithObjects:[url objectAtIndex:2], nil] withRowAnimation:UITableViewRowAnimationNone];
[self. mytable endUpdates]; // imageLabel.image=[dict objectForKey:[appSmallLogos objectAtIndex:indexPath.row]];
}