Click "Profile" on the top bar. You will see a gray link that says "Avatar" right above your Registration Information. From there you can:
 Upload an Avatar from your computer by clicking browse and searching for a photo saved on your computer.
 Upload an Avatar from a URL by finding a picture of you located on something such as photobucket.com and copying the "Direct Link" into the box.
 Link to an off-site Avatar . . . but I would stick to either option  or