Well, you need a program to create the actual image, like photoshop, and a .dds converter. Then you just create the desired image in photoshop and convert it to .dds and put it in this folder: Combat Flight Simulator 3\aircraft\shared
make sure you get the name right, it should look like this:
noseart_br_#.dds for britain
noseart_gr_#.dds for germany
noseart_us_#.dds for USA
Enter the first following number that isn't yet used instead of the #
I think that should do it.. (correct me if I'm wrong)