how do I stop Firefox doing a conditional get?

All, not really PHP related, but I figured someone here would already have solved this problem. I've got a page with a single <img>. I change the src attribute dynamically using javascript. Although the images being served all have long expiry times, Firefox still does a conditional get to which apache says "304" as it should. The issue is - I'd like this page to appear to be as "real time" as possible, and the occasional delay caused by the conditional get is a nuisance. My images are clearly cached, so how do I prevent Firefox from doing the conditional get ? There must be some HTTP header I can use. /Per Jessen, Zürich