thanks for the comment. I can exchange this very fast. I have every texture with layers in a GIMP file and can handle this. This was just my first test to find the best workflow to create textures with normal and height maps. I tested 3 methods:

1) creating textures from high poly models
2) painting textures in ZBrush
3) painting textures 2d in "The Gimp"

The first way took quite long. Second is an option that I will use for some of the future textures. The third option works best for most of the textures because it is fast and you can re-use layers very easily.