First of all this is in the wrong part of the forum, it should be in the
Forum News and Support Forum, cause it's not related to the actual game.
Anyway, if the picture you want as your avatar is on your computer, upload it by going to
Profile and copy and paste the link for the picture in to the 'New Avatar' field. If the picture is on a website just put the link for it in to the 'New Avatar' field.
You can also set the size for the Avatar in the 'Avatar Size' field; however it has a maximum of size 90x90.