Have you tried digging through the file structure and finding if they use a standard (.jpg, .png, .bmp, .ico, etc.) image extension and modifying one that is there so that on your device the "Cessna" icon would be modified by you, with an appropriate image program, to look like a B747?
Mind you, I have no idea if this would work, but it seems if the program is looking for "Cessna.ico" it will display whatever image (providing you keep it the same size) that is named "Cessna.ico" without regard to what it looks like on the screen.