That was done by Mike Schley (nickname here is schley) and he's a professional artist. That means he's not giving away any trade secrets anytime soon. What I can tell you is that he make his maps by hand drawing them, I think in Photoshop. If you look through the tutorials subforum you will find gobs of tutorials on drawing mountains but none on emulating that particular style. It's all about practicing with your tablet. You've got a solid start here so keep at it and try out some tuts and see where you stand a month or two from now.