Just one additional note. It often helps to first try to place the tiles manually, without using the Terrain Brush or Automapping features. After fixing up your image to be not scaled nor strangely cropped and playing around with it, I found out the set is meant to be used as 8x8 tiles and can be placed using the Terrain Brush as a Corner set:
Now, the Terrain Brush will not be able to place the front side of the wall (the part above is just manual fiddling). This is something you can do with Automapping. See the RPG Cliffs example in the documentation.