You will need to register but once you've uploaded your pic/s to the site you can copy the 'public url address' of the pic, click on 'profile' in the black band across the top of the window (In here you can change a lot of settings), and paste it straight into the 'I have my own pic' box (tick the little check box too). The file can be a jpeg or a gif (moving pic). If you don't have one there are lots of dedicated avatar/icon sites.
You can also specify the size of the avatar....but I'd recommend not much bigger than 80x80 cus it kinda messes up the boards if your pic is huge. Best size is 60x60 I'd say.
If you have any further questions on avatars only please post them in here