Can Safari 3 be forced to cache a large jpeg with PHP headers?

I am creating a touch-screen kiosk application, using a full-screen version of Safari 3.1, and was wondering if there's a way I can force Safari to cache a large background image JPEG. What I'm finding is that Safari 3 will sometimes cache my large 1.1 MB background image (1680x1050), and display perfectly fine, but on occassion Safari 3 will "think about the cache" and Flash the screen white for a millisecond and then draw the screen. Firefox doesn't seem to have this problem, so unfortunately this is a Safari 3 only issue. I really only want to cache this ONE image - - - nothing else. Is that possible?