Wow, a map on your very first post! Have some REP for doing that.
I'm not a GIMP guru, but your map has some strong points. The volcano definitely looks like a volcano with elements of height in the shades of brown. Also the sandy coastline stands out well against the green interior landscape.
Did you use a bump map on that volcano? I think you could increase the effect that what is shown if do (or increase contrast if you did.)
The colors look a bit saturated - but that's a matter of personal taste I suppose, so small nitpick. I think if you subdued the colors slightly it might improve IMO.
Is the a grassy or forested island? If forested perhaps use a darker green against the grassy ground beneath.
Again, thank you for posting a map in your first post!