It may help to share what program you're using, since some can't save a grayscale-mode image from what I've heard (though it be in shades of gray, it may not be grayscale mode). That was what crashed my game when I first tried to use Map Maker... I was using Paint, which can create the terrain in every other way but not in grayscale mode. Now I use GIMP for my map terrains... at least in editing them.

If all else fails, the problem could be the file itself. It could be corrupted... this happened with one of my own heightmaps, which I have yet to recreate. The only way to fix a corrupted file is the remake the thing in its entirety in a fresh file.
Hope this helps.
