Welcome to the board! That's a very nice start; I like the overall distribution of the elements (though perhaps the lakes so close to the rivers should be connected to said waterflows).

I think it would be both a good excercise and an addition to your collection to keep working on this map with GIMP. One option to easen the pains of adapting it would be to simply keep the continental shape and redraw the interiors in separate layers. Though it will be slower at first, the degree of control it gives you over the different elements is really a huge boon.