Hey arthur, welcome again.

Some things to consider...

You've got some nice land shapes but they're all rather evenly spaced and horizontally in line with each other, which feels a little unnatural. Also, I'm not sure if it's a mistake, but your rivers seem to be different colours? I thought the darker lines were roads at first glance, but I don't think they are?

You might consider adding some extra features like mountains and forests. If you're not sure about drawing these yourself, you can download brushes (just google Gimp mountain brushes). Also, a scale would be helpful, so that we know what size the islands are. A border and map title would add appeal too. Finally, I think your map would benefit from having some texture or shading on the land and sea areas. The colours are quite flat at the moment.

It's a decent first map which would look great with a few tweaks. I think you might benefit from working through a few of the Gimp tutorials at the Guild, just to learn a few tricks. Take a look here.