If using GIMP follow these guidelines
513x513
PNG
One layer
Greyscale
No Alpha Channel (Can be cleared out in layers option by right clicking your height map layer).
Also go ahead and create your map in FH. For instance, go into map maker and save the name of what you will be calling it.
Then you can save your PNG file into FH>Media>Terrains>(Your Map Name) In its folder. Will generally help along process.