Just to add my 2 cents: The avatar has to be, what, 80 x 80 pixels? The creative commons page states “No Derivative Works means:
You let others copy, distribute, display, and perform only verbatim copies of your work, not derivative works based upon it.”
To me, reducing the size would be a derivative.
What does it mean in everyday life about using airliners.net or jetphotos.net? It would be illegal but the chances of getting caught would probably be slim. The original photographer may not even recognize his photo due to the reduction in size and cropping. However, I wouldn’t create an avatar out of photo from either of those sites because the pictures are copyrighted.
Why don’t you find a picture you like, create an avatar, and then contact the photographer. Show him the avatar, explain where it will be used, and then ask permission to use it. Unless the guy is really a hard head about it, you probably would get permission. I know I’d allow any picture of mine to be used if the above procedure was followed.