This is a common problem. The easiest way to go around this is to set a random parameter on the image... like this:
<img src="lala.gif?<? echo rand(); ?>">
that way the src for the image would look somethign like this:
<img src="lala.gif?0.876546584">
where the number is a random number
make sure that you set a proper seed with srand() first.
/Alex