Avatars are set in your Account Profile or Settings. You can go to your Account Settings in two different ways. The first way is using the Account Settings link in the upper right of the page.

The second way is to use the INFORMATION menu and go to My Account and then select Account Settings.

Now go to the About You tab in the Account Settings dialog.

Go to the Browse button on this screen, next to Photo:. This will allow you to pick a photo on your computer to upload as your Avatar. The only restriction is that the photo must not be larger then 115 picsels wide.