Your right it's called an avatar....
You need to create a small image (gif or jpeg) that is a maximum of 120x120 pixels and no more than 6kb in size.
Then click on "Profile" at the top of the forum, scroll down to the bottom and you'll see the avatar settings - Click on "Browse" to find the image on your computer and select it, then click on "Submit" to upload it onto the Paddleducks forum.
I hope that helps....