a .gif image. for example, search animated gifs on google and any of those images will work if you want a tutorial on how to make an animated one or simply want someone to make one for you, just ask me
You can use Adobe Premier Pro if you have it and make a video and export it from the program as a .jiff file. Then upload it to imgur.com and make it your avatar.
There are a lot of ways to make animated avatars... Rabid is correct, though; animated images are .gifs. At least, that is the only kind that should work on the forums. In some cases .pngs can be animated, but I don't even remember how to achieve that anymore. xD I used to make very crappy animated images in PSE2.0 and PS CS3. It might be possible in GIMP and other free programs, but I haven't tried. There is one site (I will have to try and find it again...) that allows you to upload several images that'll turn into an animation, too. My friend might still know a link to another website that will take a specific timeframe from a video and turn it into an animated image, but it won't crop it down to a usable forum size.
Ok, youll have to wait a bit :\ i need to do adinec first, and I cant even start his until tommorow due to my stepfather being satan, so your right after that