With a Paint Programm ??
Paint.Net
Gimp

I see you use the "Mystymood" Blendmap. There is a new Terrain Shader in the "mtlFX.c", easy to use. Add severals Skins to your Terrain, for each material one, where is the alphacannel the Blendmap. Quadraxas also make a Tutorial about that, but i don`t know if it is finished already or Shaders are included.
Terraintut