lucy24 - 5:26 am on Sep 7, 2012 (gmt 0)
Didn't I just see this identical question somewhere else?
Oops, no, sorry, I'm conflating two threads: One of them described a closely analogous problem with CSS. That one had a simple alternative fix. The other one was me blathering about mod_expires, and I think that's what you need. Assuming for the sake of discussion that you're in Apache, because I don't speak IIS.
Your wording is a little confusing but I think what you're saying is:
Because of the way your program works, each new image gets the same name as a deleted previous image. You need the browser to reload the image, even though it "thinks" it's already got it.
Can you make an htaccess file? Do you have control over where the newly created images live? Optimally you'd put these special images in a directory all to themselves. Then you toss in a teeny little htaccess file that says simply
ExpiresByType image/png "access"
... or image/jpg or whatever it may be. The word "access" means "right now". Result: the image doesn't get cached. Browser has to reload it each time.