Go to your user CP and under avatars you will see Browse...you can then upload a picture of piece of clip art from your HD. Then all you have to do is save. There are specific dimensions (that I can't recall offhand) so make sure your choice fit within those boundries.
There is only one to choose from that comes from this site...thats the one you have.
If you want anything else then you have to pick one that you found on the web or took yourself. You save that to your hard drive and then click "use custom avatar" go to browse and find wherever it was that you saved it on your computer, upload and save.