WOW Model Viewer
Go there and download WoW model viewer. The program lets you view all game models, including characters with user-defined equipment in any animation.
One of its newer features allows you to make an animated .gif of whatever you are viewing. It takes a bit of experimentation with the output settings to get the desired results, but it's a pretty nifty feature, and quite a step up from manually screenshotting individual frames of animation. You can also change the background color to match whichever forum you wish to use the avatar on. After exporting a .gif, you can mess around with it in something like Adobe Imageready to compress it. Have fun.