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Find a picture that you like, right click on it, "save picture as" (pick whatever name you want to save it as)
Then go to My Account, Settings, then scroll down to your Public Profile, browse for the picture you already saved, once you find it, click "yes" or "upload" or "save" (I don't remember which, but that last part will be obvious when you get there)
Finally, press "Update Profile Settings"
You should be good to go!
You can also use animated gifs or blinkies.
Here's a link to some sample blinkies though some of these look too big: http://www.pinkyblinkies.lunarpages.com/Pinky%20Blinkies%20Page%2021.htm