Hmm. At first I thought it was just because that map is a lot larger than the other one, making the lines of the thumbnail harder to see (that is a part of it), but upon going back into the GIMP file and checking the colors and saturations, it is indeed a bit off. Thanks for the heads-up guys! I think it is because I was sick of clicking so many times to delete the color for each island or lake, leading me to just use color erase and go over everything with a huge brush, but I have just now found a much better way to go about things (will have to change the GIMP tutorial to reflect that). I think that this better method should also leave more darkness around things, making it easier to see the features of the land. If there is still an issue, I can always beef up the contrasts though.
Given the new method (and the state of the GIMP files I now have saved), I think I will have to start from scratch on the new maps. It should be a bit faster now that I know better how to go about erasing things, but there will still be some delay. I'll let y'all know when the new files have been uploaded though. In the meantime I'll leave the low-contrast ones up, in case anyone finds them sufficient (but know that better ones are coming). Thanks again for catching my mistake guys!