I like the shape of the landmasses. As to how to proceed, hm, well if you are going for the photorealistic sat style then I'd recommend using the cloud layers (rather than GIMP colored version you are currently working with) as the base height map. The cloud layers provide the kind of texture one wants in a realistic looking map. From there you can use GIMP's "Bump Map" or "Emboss" features to give a three-dimensional appearance to the texture, and from there it's just a matter of adding color. Since you are a GIMP user I'd recommend checking out Gide's GIMP translation of Ascension's Atlas style as you may find some useful techniques for bump mapping and coloring there.

Cheers,
-Arsheesh