Yep BuNN is right, you can chose one from the list in your USER CP, but its restricted until you have 100 posts... the more posts you have, the more options you have
To find the avatars, click on your USER CP button (at the top of the page under The cat site heading)then click on Edit Options, right at the bottom of that page is a bit about avatars....
Kiwideus, you can have quite a few after 20 posts.
Go into your user control panel and then click on Edit Options. Right at the bottom of that page is a link to the avatars, click on it and pick one of the cool ones there