You can add an avatar after ten posts on the forum. Information about requirements about avatars can be found here, in Forum Guidelines, in the "Information" bar on the top on the Forum:
Q: What is an Avatar and how do I get one? A: Avatars are the small graphical images that you see under a poster's username. You must have at least 10+ posts or have a Premium Bundle subscription in order to upload a custom avatar. Q: What is the size limit? A: 384 x 384 pixels (Square) Q...