First I adjusted the levels on the background layer. Then I found a rock shape I liked, selected it, contracted it by 2 pixels, copy/pasted into the rock image that I liked (the one above), saved the selection and discarded the original rock layer. I then copy/pasted that rock using the selection from the other rock and copy/pasted that on top of my background layer. Or you can just select a shape you like using the selection tool.
I then adjusted the levels on the new Rock Layer. The colors on the bottom tip were too yellow and looked odd so I selected the area, copy/pasted it to a new layer and adjusted its levels. I then Merged Down (Ctrl + E or use Layers Menu) all the way to the Rock Layer. I locked the Rock Layer.
I loaded the Rock's selection and pressed Ctrl + H to hide the marching ants. Then I began to shade the rock. I chose ground, plant (near bottom) and sky colors (near top). I set the Modes to Multiply and adjusted Opacity (on your Layers Menu). I then made a small shadow for the rock on the ground using a selection with a 3 pixel feather. I merged all the Shading into one layer once I was satisfied. You can then click the layer on and off to see the difference.
Rock - http://letsjapan.wordpress.com/galleries-photo-essays/images-from-india/
Death Valley - http://commons.wikimedia.org/wiki/File:Death_Valley,19820816,Desert,incoming_near_Shoshones.jpg