@A48Racing - You do seem to be wanting to run before you walk. So just enjoy playing the game, adding a few mods to learn the file structure and, trust me, it will soon become clear and second nature.
I started by playing around with the decals.ini to put stock decals where they shouldn't go but this game/sim really took off for me when I taught myself how to make my own decals and skinning soon followed although I often save time by overpainting national insignia and adding my own either directly onto the skin textures or by decals. You'll learn a hell of a lot just by downloading a template(s) (like sundowner's Phantom templates) and seeing how they work.
GIMP is absolutely fine to start with. Indeed, 12 years later, I've never felt the need to move on to something better and for decals it is quite simple and very fast if you set your default image (in preferences) as a transparency.
https://combatace.com/forums/topic/76701-making-decals-using-gimp/?tab=comments#comment-609704