The points these floofs above me here have given are valid. You may also want to possibly try .jpg format if you're having problems with .png. I know with the newer GIMP, you have to export an image in order to make it into a .jpg, .png, or other format other than .xcf, which if only saved it will automatically format to that. Just check your files and make sure they're being saved correctly.
Update as I was writing this post: Just tested your heightmap in my own game and got the same result which was a crash as soon as the name was entered in (I saved it from here to my files and called it TestTerrain.png). Tinkered with it in GIMP and re-exported it with a disabled Alpha channel and it worked. So, double check and make sure the Alpha channel is removed, which can be done so by right-clicking on a layer, and selecting "Remove Alpha Channel" in the menu that pops up. Then export as .png or .jpg. And don't worry, I've already deleted your map from my files, so that I'm not "stealing".
Update 2: With the older versions of GIMP, like 2.6 and lower, you can save an image as another format, no need to export it. But anything above 2.6, such as 2.8, requires that you export instead of save.
Good luck to ya floof!
