By pressing and holding CTRL L on your keyboard you are able to remove the alpha channel (if there is one) through your layers menu. Right click the heightmap layer, which should be the only one there and there should be an option called 'Remove Alpha Channel'. If that option isn't available then don't worry, there isn't an alpha channel to remove.
With the later versions of GIMP you have to Export the image as a .png instead of saving it. Are you doing that? A message might appear asking you to export or flatten the image. Click Export and a box like this will appear:
Click Load Defaults and then click Export again. Make sure you're saving it in your Feralheart >> Media >> Terrains >> MapName's folder. When you load it into Map Maker make sure you include all capitals, spaces and symbols. Whatever your image name is just type it in exactly and don't forget the .png at the end.
