This is a simple photo-tutorial for anyone who would like to make decals using GIMP.
After using GIMP to get started I soon tried to 'upgrade' to Photoshop Elements but quickly came back to GIMP because it's just so damn easy! Seriously, it's enabled a ham-fisted amateur like me to have Voodoos in RAF service and Lightnings in USAF service! For anyone with an interest in 'what if' making your own decals opens up a lot of possibilities - lol, it's kept me busy for 10 years. For example, you may be Venezualan but just wish that your government had bought Phantoms or Harriers. No problem! We can put that right!
So, for now, forget about 24-bit, 32-bit and Alpha channels. This will be enough to get you started and you can learn the more advanced stuff later.
Before I start, I should point out that this tutorial is based on GIMP 2.6.11 but I don't think that the recipe is any different for later versions.
As explained, for making decals it really helps to set your default image as a transparency (Photo 2).
I do hope you can follow it and sincerely hope that I haven't cut out any important steps due to my familiarity with this amazing free, open-source software.