Ok.. so your are looking for the more ISO looking mountains and stuff. Start by taking a look at
this map. The artist did a nice tutorial which I LOVE (thought I have yet to have time to make a map in ISO style yet). Go to his site
here, Click on the Swap Menus "image" right above the scroll on the right hand side. This changes the icons above the Swamp Menus icon and look for the Workshop icon. It looks like a green jar with paint brushes sticking out the top. There is a very nice tutorial, and though it is in black and white, the concepts can be used for color drawings also (mainly using the smudge tool to make nice mountains and other color blending effects.)
Also, view this
thread for a link to an offsite hosted Video tutorial on making some ISO mountains.
Either of these could be done in GIMP or Photoshop with some slight variation in technique.
Hopefully, this is enough to get you started.