I believe custom avatars are restricted by community rankings. Before they made changes to the rankings, you had to have 500 posts before you could have a custom avatar. However, after the change, you can get one sooner, though the point at which that's available is uncertain. IIRC, I've seen a couple with custom avatars around 400. It's more than posts. It also encompasses logins, posts read, minutes online, etc.
ETA...try clicking on your forums settings and click on the avatar tab. You might see an option to upload the avatar from the web. I know my options are different due to the time on here, but you might see an option already.
