I've had this problem...probably twice I think, happened recently too and to solve it I reopened a new page in Gimp, double checked it was greyscale, double checked the size was 513x513, and redrew the map and it worked. There was nothing wrong with the old maps I used but it refused to load it so I checked everything and redrew it - seemed to work.
Also, I don't know if you did this but do not make maps in MS Paint/paint/paint shop pro etc. because the game always has issues with them.
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Another point about map making, for some reason exporting maps causes fh to mess up. I don't know if that was sorted but often when you export a map and close fh, then re-open it, before the KovuWorks pic appears, fh crashes. Not sure why but fh+ can export maps for you if it does this.