Based on the FAQ it looks like we don't have access to upload a custom avatar currently.
What are avatars?
Avatars are small images that people use to identify or distinguish themselves to other forum members. In some cases the administrator will not allow avatars or only allow them for specific members. The administrator can also provide some stock avatars people can use, or they can allow members to upload their own custom avatars from their own computers. These avatars will be displayed as part of the user info in posts, as well as in the public profile.