If the image you are showing is the image you are importing you probably need to use the scale function in TESAnnwyn. Going from black to white is to much height difference for Skyrim, it can only handle 1/4 of the whole color range. If I remember right -s should be used before -h. And if you are not changing the height, you don't really need to write -h function at all. To me it's always a bit tricky to know how much to scale, you just have to try and see what works, it all depends on how your height map looks. I have never worked with a 64 bit image, are you sure TESAnnwyn or Skyrim for that matter supports that high bit range? Well other that that, the command line looks like it should.
