can't change my avatar

[*****Sniper]

I am a new member of this forum, i would like to add an image to my username, but when i go to the "edit avatar" section I cannot change the "do not show avatar" button! can anyone tell me why that is?
also O am planning on buying vBulletin, is it possible to change this and allow my future forum users to post an avatar whenever, however they like?!?!?