I forgot about the file size deal. From the avatar page, " Note: The maximum size of your custom image is 100 by 100 pixels or 48.8 KB (whichever is smaller)."
Which means if you are planning on having an animated GIF avatar, the overall pixel size needs to be reduced, as the more frames added for animation, the larger that KB size will be.
If you have an animated GIF file you wish to animate and also size down to suit the forum requirements, you will need a program such as Adobe ImageReady (Additional program which came with Photoshop CS) which specializes in web related graphics. Open your GIF file in ImageReady and you will see each frame/slide of the animated GIF appear in seperate layers and columns (A breakdown of the animation)
If you open up an animated GIF file inside Photoshop all you will get is the very first frame, flattened and indexed into probably 256 RGB colors..... basically nothing to work with. If you plan on cropping down the size of the GIF to suit the requirements here of 100x100px you will need to convert the file from Indexed to RGB so that Photoshop/ImageReady effects and layer styles can be applied and not limited to just 256 colors (Which makes GIFs smaller then JPG files in general.)
Once you have applied the effects or transitions you wanted and for how long, export the ImageReady file back into a GIF and open it up. It should work pretty decently.
Note: For animated avatars for most forums, you can only get about 3-4 frames at the most before your filesize starts to jack up fast. You can either reduce the overall size of the avatar (Thereby reducing the amount of pixels that need to be calculated per frame inside the file) Or reduce the amount of frames used in the GIF. (A quick method is to delete every 2nd frame.... it'll make the animation seem faster but it'll be even and smooth.)
Or if all of this is way over your head, do a Google Image Search for "Animated Gif" and filter the results to display only "Small Images." ~ You might also get away with Medium Images for things above 50x50... but I think you'll be hunting longer that way.
Enjoy.