According to Jeremy:If it's not related to the above points, it might also possibly be an issue of the image being too large and needing to be shrunk by the forum to fit the now mandatory 100x100 and losing the animation.
Jeremy said:Fortunately, we will be able to bring back the animated avatar add-on and it's even easier to use than before. You will no longer have to worry about resizing your image before uploading it.
The size of the image has nothing to do with it. It may have just messed up when the forum software was converted to XenForo. If you have the avatar animation add-on set to active and you re-upload your avatar it should work just fine!