You have to darken the LandTile images. It's the same in the original files. If you press Ctrl L in Adobe Photoshop, adjust the level in the table from 255 to about 230. You have to feel it according to the given image. This way you only darken the LandTile files. Do not edit the texture. And in the game you will know it by the fact that flat terrain is landtile. Shaped terrain is represented by texture.