Here is my first attempt to create a world/continental map using GIMP.

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I followed this tutorial and found it pretty clear. I didn't finished it yet (and I think I made a mess with bump mapping when I added relieves, since the whole continental part is bumped up! ^_^')
Despite the work is highly in progress I already have some doubt about the procedure. I think this method has some flaws that prevent to achieve good results. I'll post my consideration after diving in much more tutorials. I'm sure all issues I've found have already been addressed thousands of time, so don't take me much seriously. I still have to learn a lot and I'm writing here just to clear my mind. In the meantime, every feedback is welcomed!

Many thanks to all!