Here is my GT6 photo thread, I'll try to keep this thread updated and be active in the GTPlanet competitions. Hopefully you enjoy looking at my pictures as much as I did taking them.
Obviously it's not very good but it was just a test, I painted the car white and took a photo. Then on GIMP I cut around all the none white parts (windows, lights, background, etc) and put them on a separate layer to the car. I then clicked "colour to alpha" on the car layer which deletes the white (or any other colour which is there I assume) but leaves the shading/highlights as an overlay, then with a blank layer beneath you can make any livery you want!
Nice gallery with some very original shots. The first KTM shot, the close shot on the hand of the driver or the last Nascar shot are very good with some great angles and point of view.