take a picture of your buttcrack and upload it. don't know how good it will be but i guarantee it will be good for a few laughs hehehe
I love that tiger pic, its really beautiful and magnificent, and cool all at the same time.
Find something distinctive that you like. If you are good with art you could make your own.
To actually make it you will need to use some kind of drawing or picture program. Select the portion of the image you want to use then save it as a separate jpg file. Upload it on your profile page. Keep in mind that avatars are small and don't show a lot of detail.
You could also find one on the web somewhere.
Mine is a closeup of a shirt I'm wearing.