I've found that the very best way to make sure you get replies is to continue to work on it even after you post it. Don't sit back and expect everyone to find all the little flaws. Take a 10 minute breather, flip the image horizontally, and look for problems yourself, then spend a few more hours on it and post an update. It's an acceptable way to bump it and it improves your ability to look at your own work objectively.
Edit: Err, scratch that, don't bump it if it's only been a few hours, that came out wrong, just edit the update into the body and the subject line. I can't speak for anyone else, but I know for me if I can see the original image and the latest revision it really helps me come up with comments. Also, I don't find it at all illogical to occasionally bump your own thread if it's dropping off the page and you still need more help. There's quite a bit of activity here and posts just sometimes get skimmed over and forgotten.