I haven't worked with GImp yet, but I heard of two features/plugins/filters/whatever - one to make textures automatically (?) seamless, and another one for creating grey-scale heightmaps. Has anyone of you used these and know if they are good or not?
I finally got it installed, rather unconventional but it works.
I'm quite surprised by it, it's not PhotoShop, but very powerful indeed.
Actually I also wanted to have that seamless feature, too. I used it years ago for homepage backgrounds (iiiieks!! MANY years ago!!) but I don't remember the program, maybe it even was on my old Amiga computer. I just played a little with it and... well, for nature backgrounds it could be quite useful, but it uses transparency effects and I guess for some texture it wouldn't look good.* But I gave it one of my own textures which were not that perfect before and it looks great now. I guess you'll just have to try it out, it's difficult to describe. (Editing it manually is not that hard though... just changing the offset, smoothing or painting the break line and reset the offset again.)
I'm not sure about the heightmaps... there even is a filter called "Bump Map", but it doesn't seem to be what it promises. But it's as easy as with PhotoShop though, just convert to greyscale and adjust the colour values (even inverting works that way).
Give it a try, up to now I really like it and - it's free.
EDIT: * and you can't adjust values for this effect.