How d'ya like that, no HTML... let's try this again...
I have a suggestiong but first one question: WHY?!? would you not want to cache your images??
if you have a good answer then:
when you output your img tag:
<img src="yourimage.gif">
put a RANDOM... long random... as near to true random string at the end of the .gif..... like:
<img src="yourimage.gif?averylongandUNIQUEphp4string">
That way, when the browser receives each image, they are UNIQUE and immune to caching....