No problem! It looks a bit better but it's still worth checking out that tutorial. It's not too long and explains several pretty basic rules. One of the biggest ones I think applies to your current map is that rivers tend not to fork out much. If there is a fork, they tend not to go in completely different directions. Lakes in-land tend also to have only one river leading out of them. It's all explained in the tutorial, and won't take very long to read.

As for the borders of the map, I'm not familiar at all with GIMP. I've always used Photoshop, and should probably check out GIMP sometime soon as there are quite a few tutorials for that program on this website that I'm not sure how to replicate in PS. How to apply more detail to the edge of your current map also depends on how your current file is built up, in terms of layers. I tend to go overboard with layers, but that makes the filesize kind of big. Plus, the shape of the land itself is usually what I start with, like with my current map:

Attachment 65192
Attachment 65193

Start with big blob, then use a much, much smaller sized eraser brush to carve out the edge. How much detail goes into it is up to you, but like I said: I'm not familiar with GIMP so I don't know how editing what you currently have would work, exactly.