Avatars are small images that display alongside a user's name. This is a feature that is reserved for established users on the MacRumors Forums. We require a 500 post minimum (a user title of 6502a or higher) before you are given the opportunity to upload your own custom avatar. A few users were given avatar privileges before this milestone because they won a forum contest.
Once you reach 500 posts you may not immediately be able to upload an avatar. The privilege is added automatically when the forum system does routine processing, so check again in a few hours. The delay is normal so please do not contact us to report it as a problem on the same day that you reach 500 posts.