I had the same problem when I made our nose art.
I developed a work-around and started with someone else's noseart that had a known good 'clear' background. I tried using the numbers given in the help file, pasted below, and never quite got it right that way.
fuzeman
[ heres the stuff from the help file, under squad info ]
The Upload Noseart button can be used by the squad CO to upload a 64x64 noseart graphic. Graphics are not displayed in the game nor on the web page until they have been reviewed by the HTC staff. The transparency color to use in order to make your noseart blend into the plane is {127, 127, 127}. To upload a graphic, make sure your 64x64 bitmap file is named with your Game ID and is in your ../HTC/Aces High/squads/ directory. Next, click the Upload Noseart button. Once the noseart is approved, your squad members may force the new image to download by deleting the existing noseart in their ../HTC/Aces High/squads directory.