I'll try to find out how to do it in GIMP. I don't really understand all the features and what they do yet, however.

My intention was actually to make them thinner, like the example map I linked to, and to have some sort of systematized spacing (it's kind of random as I have it), but making the map big enough to do that would have crashed my computer. I think I'll split the map into quarters, resize those, and redo things like text (I can't get names on the rivers in the size I want because of the image size, and the mountain range names are all screwed up. It's related to the italics, I think), the coastal rings, and the rivers.