One thing to do is load it into MS Paint that comes with Windows and save it as a 256-color bmp. You could also Google-search for "Bright", which I have as a plug-in for Photoshop. It indexes the colors and it really helped my noseart look good in the game.
Have you looked at the noseart in the game offline? That way, you can test to see how it looks on various planes/vehicles and tweak it until you're happy enough to submit it as the CO of a squad.