The file that you have tried to attach is too big. The maximum size is 10000 bytes.
So I'm thinking, ok, I made sure it was 100x100 and less than 9.8kb, but I still get that message. I saved some other avatars on the boards to make sure I was getting the size right, which I was and a bit lower to boot, but no go. Heck I even went so far to make the avatar 7x7 pixels and it still gives me that error message.
What's going on? Is there some more waiting time for us lowbie posters before we can use this feature?