Height maps must be;
PNG File
One layer
Grey Scale
No alpha channel (Just keep greyscale entire time and shouldn't worry with this. However once you follow all my instructions and it still doesn't work. Go to the layers tab in GIMP and select your layer with right click and click remove alpha channel.)
513x513 pixels.
Also be sure to save the name of your map first in Map Maker. Then when you pull up GIMP and make your file, save it in FH>Media>Terrains>(Find the folder with the name of your map) and save it there.
Also, name the file something like FirstMapTerrain.png
Hope this helps.