You can also load images as heightfields, if you want to use them as elevation data. The Image Map Shader can create displacement, thus creating a terrain, but you can also load it into a heightfield if you prefer to work with those. Simply use the "Add Terrain" button, select "Heightfield Load", and you can choose an image. Many common image formats are supported, even if they don't show up in the list of file types. Change the file type to "all files" so that you see all your files.
Matt