Catmommy, all you need to do is go to the "userCP" button which is just under the "The Cat Site" title, click on that, then click on where it says "Edit Options" go right down to the bottom of the page, and on the left hand side it says "Avatar" - click on where it says "Change Avatar" and your options will come up - just pick the picture you want!
Also, the more posts you have, the more avatar options you get. At the top of the Lounge is a thread that answers a lot of other computer questions too. Check it out!