Thanks for your comments!

@ Madcowchef & Lingon : yes, I wasn't really convinced and you pointed the "why" very well! I may try something else... I don't think I can do what you described MCC (thanks for the explanation), and I can't use the good tutorial of Diamond... or don't know how to convert it from Photoshop.

@ Raptori : it's a combination : mountains done in Wilbur, then a filter "liquid drawing" in Inskape, then a few corrections in PSP. With two layers, one with a lot of mountains/hills for a background, one for the major mountain ranges.