In making sure that there is no copyright problems I've looked into the non-profit use of people using dorianart.com graphics in avatars (and other means). It has been requested that people please give credit to
http://www.dorianart.com when using them. How you do so is up you, either as a sig, in your profile, or in the graphic itself.
For those who don't know, Dorian Cleavanger has some incredibly fantastic artwork at his website. I highly recommend everyone check it out. Some originals are for sale and signed prints are available for very reasonable prices. I've placed a link to his site on my main page.
Using any as a avatar is purely up to you, however, you should host a small version of your choice on your own webspace.
http://www.dorianart.com
-Ryandor