Avatars are small icons that you can specify which help identify you graphically. If you have an image you want associated with yourself as you use the forum, you can change your avatar by going to the control panel.

The control panel is a link at the top of the page after you log in. It says (User CP - Log Out)

Click on "User CP" and then on the left-hand-side you will see "Change Avatar"

The maximum size is 20x20 (I know that's very small!)

You can see an example in the "merger" thread from Terry (Lawrence of Arabia)

-Michael