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I believe that would require some sort of Javascript or Flash which is not enabled for users, so on, what i have interprated from your question, it is not possible. You can always have an animated .gif but that's the closest you'll get.
Sadly what you're asking for is impossible without Javascript, and members can't embed HTML or Javascript into their signatures. Heck, even though the Admins CAN do it, they WON'T do it even on their own signature since I think (you may correct me if I'm wrong, Staffies) that's against some rule that the Staff members have to follow.