Hi, Arthurius!
That is really, really good!
Stock textures are pretty useful- with the exception of Kulthea- I just used the stock planetary maps that come with Celestia, for the rest of the planets and moons. (The Celestia Motherlode is a good resource for additional textures for various celestial bodies.)
Certainly, feel free to use the images I've posted at flickr, in anyway you see fit.
Got to admit that I've never tried GIMP- I tend to use Photoshop v6- although I do like the bizarrely named 'Project Dogwaffle', which was heavily based on Deluxe Paint for the Amiga. Version 1.16 used to be free, and supported all sorts of high-end effects like optipustics, which was great for creating fur, trees, etc.
Keep up the good work!
All the Best,
Kevin.