The second post in this thread tells you how to edit your profile and insert the gif there, then it will play. That's all you need to do once you have the gif loaded into linkable webspace which can be done via the instructions at the top of the image forum (you only need to use the load part, not the post part, but the post part is exactly how you will do it when you go to edit your sig, as it works the same way as posts to a forum with the vB codes)
Myriads