I am going to go over everything you should do.
Make sure it is Grayscale
1 Layer only, if you have more you should flatten it, even sometimes flattening doesn't work though.
513X513 or 512X512
Remember to put Terrain at the end of the file name, for example: worldTerrain.png
Make sure its .png
Make sure its in the maps folder
Hope it helped, although I know you've tried most of these :3