^
What she said.
But here are a few things that may help with your heightmap:
1. Try saving it in a new format and trying that.
2. Try selecting all of it and copying it. Then close out your image program and then re-open it so it's blank. Then press Ctrl and V on your keyboard and paste there. Then flatten it, then save.
3. Try changing the size to 513x513 or if it already was 513x513, make it 512x512.