Hi USAREC!
In Aces High you MUST use BMP files, although they, normally, do not allow transparency.
Get any bitmap editor, like, Paint Shop Pro, GIMP, Photoshop, etc.
Load your GIF file, then save it as BMP.
Reload that BMP file and make sure that the color that will be transparent in Aces High has 176,176,176 values for RED, GREEN and BLUE color components.
You will not see any transparency for that BMP file in your editor, or browser.
However, that color (176,176,176) WILL be transparent in Aces High, which shows files in its own way.
If you want to have a preview paste that nose art on a background, with same values.
When you're satisfied save the file in your \Aces High\Squads folder. Go online and choose any bomber(that will allow you exterior view). Go to the runway press f3, for example, and choose best angle to admire your masterpiece.
When you're finished, tell your sqaudies to delete old noseart, to be able to use the new one
I just hope not to see that noseart on my 6.......

Sorry for long post, take care!