go to your "USER CONTROL PANEL"
click the "PROFILE" tab
select "EDIT AVATAR"
click "BROWSE" (window opens)
scroll around to find the pic you want to use, you may have to back out of folders to get to the right file, click on the pic you want when you find it (window closes)
click "SUBMIT"
if the pic you want to use is too big, it wont work. in that case i open my paint program and edit the size. the maximum size you can use is 80 x 80 pixels, i always use the stretch/skew to change the size, then you can look in attributes to see the size (pixels), if you manually change the pictures pixels it will crop the pic, not shrink it.