Hi,
Matt GIMP is not the best option for digital painting, you can do it with it but its not so well, there was/is a project called GPS(GIMP Paint Studio) for it, which adjusted the GUI for it but today the developer is part of Krita team, after he worked for mypaint. Even photo retouche is a broad area, you have to pick a tiny bit for it and mostly you have to pick something with "WOW factor"
I suggested a few days ago making a tutorial row for the Ambassadors and doing a so called "Global Wallpaper Hunt" for F24, looking to this Sky Replacement would be a fine topic and this way the people can pop up their holiday pictures.