I was inspired by this animated avatar from free-avatars.com:
Is it possible to use an animated GIF as an avatar on gravatar.com? I couldn't get it to work, but maybe someone else knows a hack.
migrated from superuser.com Nov 3 '10 at 14:55
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AFAIK, this isn't possible.
Everytime I upload an image, gravatar's system forces me to "crop" it. I believe they just extract the first frame from an animated image.
Check this blog post, you can see in the comments where people are complaining about this issue and png transparency too.
EDIT: Since there's a bounty for this question, here is a another quote from their official blog answering to a user asking the same thing (emphasis is mine):
...which is answered:
This is really impossible. I yesterday I have asked their support, and they said:
Their reply is as follows