Any Graphics Editor that can modify and save these files should work...TGA, BMP, JPG, DDS, since these will cover just about all of the editing that one would encounter with the maps.I have Paint Shop Pro and know it inside out - been using it for years. Would that do the job?
If, I were just starting and wanted to learn how the maps worked, I would just edit one of the hundreds of maps that Garnier have created.
Just adding fences and houses to any one of those maps will change the appearance of them. This would allow for someone to start altering maps without having to learn the entire process all at once.
The whole process is simple if you understand it, but LP probably said it better than anyone with the reference to an instrument.
davinci