I was asked to do several Avatar pictures. The concept was simple, take a photo of an existing person and transform them into an Avatar character.
The first one I did was of Travis Barker, drummer for Blink 182. I was provided with a great photo to start with from http://www.blink182.co.uk/
I kept most of the photo from the jawline down the same and just coloured it. The top half of the face was reworked a lot more.
I widened the head and the gap between the eyes and created an avataresque nose. I used transparent layers to paint the facial patterns on and just kept going until I was happy with it.