If you still have issues, open your finished heightmap .png picture in your photo editing software. Save it down as a .jpg picture. Load the .jpg picture that you just saved that still is the heightmap, and turn it back into a .png(make sure it's the right size), then try to load your "new" .png instead.Did this and it worked like a charm! Thanks, Red, and all of you who tried helping!
Some people have had this picture in the past, and it usually leads to them just using the wrong properties/preferences for their .png pictures, causing the game to crash.