Click on the Your Account on the left side of this page. Then click Your Files at the bottom of the page. Once your there, upload your picture you want to use (make sure you follow the avatar guidelines). Then once you have finished uploading the avatar, return back to the "Account" page and look for Avatar Image: (it should be above your signature box). Then click on the down arrow and you will find your picture in there...
If I am correct, the image can be no larger than 100 x 100 in pixel size...
Good luck and I hope that helps!