Well, to make one thing clear right away, you can't make heightmap in Paint as it seems, this always causes crashes to the map you are meant to load.
You might have put the heightmap in incorrectly as well, cause the game shouldn't crash until you actually try to load the given map. The map maker and multiplayer should work all fine if I remember right. Maybe you did something with objects as well?
If I were you I'd change back things as they were, getting files back in place and such(Can't know what you did for certain) and then get GIMP once the game is working fine for you, try to make a heightmap with that instead.
As far as I know there's no layer options in paint, neither is there a grayscale mode, these are both needed to be checked up on before trying to use a heightmap.
