Posted: 21/04/2012 at 22:21
Press "settings" (near the top left of the page). You will be taken to a page with your name at the top next to the "head and shoulders" icon which is your current avatar, and "edit" on its bottom left.
Click edit, then browse your computer to find the avatar that you want.
Click on it and it will appear on the page in the avatar slot; save and you're done.