Go to your profile, then find the link that says "change picture" on the left side. Click it and it'll let you choose an image.
It can be picky sometimes, so if you have any problems with it, post them.
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If you want a Scratch animation as your profile picture, you can request it to be converted to GIF here: http://scratch.mit.edu/forums/viewtopic.php?id=16681
Coolstuff, Kat was talking about animated avatars.
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