Oh, I can try to help you out right here, man.
Software: GIMP. I used PhotoShop in school but I don't have my own copy of the program, but GIMP is great is pretty damn good and it's free.
www.gimp.orgHere's a tutorial link I found that teaches you how to make transparent images in GIMP
http://www.fabiovisentin.com/tutorial/GIMP_transparent_image/gimp_how_to_make_transparent_image.asp(I had to read it once because I'm pretty rusty and GIMP's interface is pretty "different")
I only work with .gif's and .png's but I've heard that making transparent colors in a .bmp is a little different. Here's the best wording I've found on the subject:
To make a transparent bitmap you must make sure that the bitmap has been converted to Indexed 256 colour. 24bit Bitmaps will not make a transparent image. Make sure that pixel (0,0) in the top left most part of the picture has the colour you want to be transparent.
If you have a background colour that you do not want to be transparent put another colour at pixel (0,0), preferably 1 that doesn't appear in the rest of the image. Some image editors allow you to add a colour to the palette if needed. In some cases setting colour 0 in the colour index also can help.
So here's a transparent .png:

Making transparent colors in .png or .gif is just a matter of adjusting the threshold on the fuzzy select tool and maybe using the eraser a little bit and adding the alpha channel.
But for the bitmap I think all you need to do is convert it right and keep that top corner pixel in mind, not sure if I did it correctly...

You'll notice that a lot of the other nose art in the Aces High squads folder has some sort of gray box. My squad's, for example, has a gray box but it doesn't show up in game.
Good luck man.