Well you go file > import > terrain from height map

and that gets you the terrain height. Then you get a colourful picture and make it the terrain's skin. You can give it a second skin and it will be used for the detail, as long as it is a different size than the first.

but...

The best thing to do is use a 3d rendering program(I use blender) and model the terrain. Then you render a top view, with black fog, and use that as the heightmap, and then you put a texture on the model, and render it with shadows and fancy lighting, and use that as the texture of the terrain. That way the terrain gets good looking shadows that are actually part of the texture.