Welcome to the Guild Riathar! Well the first thing that struck me is that your rivers are doing some weird things. For starters, rivers don't generally flow from one shore to the other. They flow downhill from a watersheds and end up in the sea. At any rate, you can read more about this here. Another thing worth mentioning is that aside from color, there isn't much in the way of geographic detail; no hills trees etc. Might be nice to distinguish grasslands from forests. There are some great tutorials for creating maps in GIMP that might offer you some pointers here.

Cheers,
-Arsheesh