Hello invernomuto1976, welcome to the Guild and congrats on your first map!
It's true that mountains would never form in the way you drew them, and there is a thread here:
http://www.cartographersguild.com/sh...ad.php?t=30156 that gives you pointers on geographical issues, but let's talk mapping first.
You are correct that you used too many mountain and tree icons. Spread them out a bit and don't worry about leaving blank space, there should be lots of that. In addition to making it easier to see the parts of the map as distinct, you will need room for labels later.
The icons you chose are fine and look good. Just don't fill the entire area.
Your river appears blurry, but I suspect that is either an unsteady hand on the mouse, or perhaps a resolution that is too low. You should be using at least 2000x2000 px, bigger if your computer can handle it, and a resolution of at least 300dpi.
The river shape is good, it comes from mountains and flows to the sea with tributaries joining correctly.
Your coastline is ok, although I'd make it just a little more irregular with a few deeper bays and longer peninsulas.
The color and shading is nice.
For a first experience with both GIMP and cartography, congrats on a fine start!!