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Did you make sure it was a .Png image? And it ended in Terrain? Example; RandomTerrain.png {Don't forget the .Png!}
I had this same problem too. If you have any like say... You have a medium grey color, then put a white shape fully filled there with no gradual decline of color. Try replacing the white with a less whiter color. I did this and it started working again. c: Make sure to keep a backup of your origional heightmap though!