How do you?

Click on User CP at the top left side of the page.
There should be an edit avatar option listed, not sure if you need a certain amount of post sbefore they are available though.

In order to get an avatar, you have to go to and click "User CP" (top left), and then click on "Edit avatar". The number of avatars available to choose from depends on how many days you've been a member, and how many posts you have. That's explained in this thread: http://www.thecatsite.com/forums/showthread.php?t=46669