What about using www.nasa.gov for planetary images? As crazy as it sounds, they have very high resolution pictures of planets that can be altered, colored, or manipulated to maybe fit the profile for this "rock planet idea". Here's a link to one such gallery of Mercury:
http://photojournal.jpl.nasa.gov/targetFamily/Mercury?start=10
Let me know if you want maybe some photoshoped for some ideas let me know!