If you have the animated image saved as a .gif file, you simply upload it to an image-hosting site and insert the image's URL into [image][/image] tags like you would with any other image, and then add all of that to your signature.
It works fine in this post, and if you look below, I've managed to use it as my signature without a problem.
Nyannn -> RE: Signiture (7/3/2011 7:25:10)
I done [image]http://i756.photobucket.com/albums/xx208/123456a_06/300px-Nyan_Cat_animation.gif[/image]
It dont appear in sig
Stray Cat -> RE: Signiture (7/3/2011 7:48:04)
That would be because you didn't check the "Add signature" option before making your post. ^__^ Above the "OK", "Preview" and "Cancel" buttons, there's a checkbox which you must manually select in order to add your signature below a post. I've added your signature to your above post by doing so.
Also note that, in this case, the image exceeded the 100px height limit by 200px and was therefore removed by a moderator.
Xplayer -> RE: Signiture (7/5/2011 11:10:48)
Here's the image cropped to proper forum specifications