I was playing with that last night because you cant get a good image photoshopping peoples photos, tho you do get some realistic looks.
but i found the program too simple and limited, ok its free
i looked at redoing the bitmaps them selves, but without the plane for referance it wasnt much good, i'd like to see the plane represents in its side, top and bottom views which you paint, however you want THEN it shows you the 3D result.
you might even get closer with the programs avaialbe to modify planes on the Microsoft flight sims