ok so that's kind of easy. Once you have your drawing finished and imported in GIMP on a new layer above your background layer, just set the drawing...
Thank you guys,
@JTurner : thanks for the grammar lesson :D About the capitalization, I alreadt changed it after rdanhenry's comment :/ Btw this map...
Pretty good job so far. The colored mountains needs more work on shade/lightning though. But for now, with the overall blue, it gives the map a night...