I think Midgard hit the nail on the head. Since most of my mapping is done in Wilbur and/or Terragen, I can't offer much help in terms of gimp (or other image editing software), but I will toss out a couple ideas for your consideration.

Desert Mountains - Consider a more sandstone like color with maybe just a very light touch of red mixed in. This should let them stand-out from the dunes as well as making them stand-out from the more temperate or arctic mountains. To give them an even more unique feel, you might try creating a few plateaus here and there, that might give a better suggestion of the wind/sand erosion that has gone on for however long.

Forests - Along with what Midgard has already mentioned, you might try a bit more contrast in the greens. While shadows (bevels) and such give a great sense of texture, a slightly wider range of color might go a long way towards suggesting a good mix of tree species.

Again, just suggestions for your consideration. All in all, though, not bad for a "beginner" map. Not bad at all. Keep up the good work.

GW