There's a few things you can try to get the heightmap to work, they're very touchy.
One thing you can try is copying the image to clipboard from GIMP, (Ctrl + C) and then closing it. Press Ctrl + Shift + V to create from clipboard. Or you can do that from the menus if the hotkeys don't work:
Then right click the image layer, and flatten it.
Save over the old one, and try it ingame again.