Cryogenix, You'll be surprised how quickly you reach 100 posts. I don't remember when I added an avatar, but I was not nearly as observant as you. I didn't notice the avatar information for a few weeks. You know, it seems silly, but it's fun to reach 100 posts and become an adult cat.
Cryogenix, when people upload new (custom) avatars it takes up space on the servers, this may not seem like a lot of space, but believe me these forums are huge! and every little bit counts (and costs). So, this option is limited only to regular members who stick around long enough.