Oops - you won't until you have 100 posts.
Once you have 100 posts, you can choose one of the avatars in the list.
Once you have 1000 posts, you can have an avatar of your own creation if you wish. Avatar policies thread
<-- that's where padders (the board admin) has listed the avatar policies.
Sorry about that - It's been so long, I forgot that you can't see the avatar list until you have enough posts to have an avatar.