[tamlin]

Originally Posted by Ravenbomb

No, not rollovers. Like these things:
Hidden Images Illusion - Mighty Optical Illusions
You highlight the image as if you were highlighting text, and a hidden image is revealed. I've kinda gotten it but can't quite nail the effect down.

Aha - now I understand. I've never seen those type of images before so I've no idea how they're done, but I suspect the answer lies in the halftoning. IE represents highlighting an image by overlaying it with a halftone screen which interacts with the halftoning in the image, hiding some areas and revealing others. Firefox highlights images by overlaying them with a solid semi-transparent grey layer (which is why the "illusion" doesn't work in Firefox).

If you've already developed a partial technique, post it here and we'll see if we can finish it off.