From Online Manual
Avatars are images that members select to represent themselves online. They are shown in profiles and beside every post or personal message. Other members use the avatar to easily identify a member. Commonly avatars are abstract images, photos of celebrities, or pictures of cartoon characters, not a photo of the member.
Administrators use Attachments and Avatars Manager in their Adminstation Center, to can control who can set and change the profile, and whether they can select from a list, upload an image, or link to an offsite avatar. They can set policies on avatar upload file size.