As long as the images follows all the standards for a heightmap it should still work even if you copy it and use it someplace else.
It does find the heightmap though it seems, otherwise it would've told you the differ.
If I were you I'd double check so everything is as it should with the actual heightmap.
-Png imag
-one layer only
-513 x 513
-grayscale
Just to see if you missed something or for some reason it were changed. If you see that everything is as it should, open up the image in gimp, make it a Jpeg image, save it, then open up the jpeg you just saved and then turn it back into a png, paste it into the FH folders and try to use that one instead. Still no luck, try to change the name aswell, and then load that one.
If still no luck I wouldn't be sure actually and I'm probably missing out on something that someone else probably knows about.