If you are using the Mozilla Firefox browser, you may have animated gifs (the little pictures that move, like some smilies and avatars) set to only play once. I think this is done to reduce CPU load on your PC or maybe to stop annoying ads from repeating. However, most ads are flash or similar so that won't help anyway.

If you want the animations to repeat, so you can see them again in case you missed them or you simply like watching them (like me):

(1) open a new tab;

(2) enter "about:config" in the address bar, this opens up a screen with all your configurable parameters;

(3) go down to the entry for "image.animation.mode";

(4) double click on this entry and change to "normal".

The other options here are "once" and "off" (meanings are self-explanatory).