(Move your mouse over the images below)
(Use the Right Click Menu to view the source)

This frame shows what happens when you try to create animated menus using just animated GIF's. Each rollover requires 3 images
(e.g."home_static.gif", "home_on.gif" and "home_off.gif" for the "home" rollover)
In IE5+ and NS6+ you will see some animation. In NS4.x (and possibly IE4.x) you will only see an image flip.

The reason you only see an image flip in NS4.x is because, as the images are cached, the animation is played "in the cache" this means the animation is finished by the time you display it, and it never restarts. The only way around this is to not use image pre-caching but that means there will be a delay when you mouse over as the new image is loaded.