I think many of the people here use something like Fractal Terrains or Wilbur because they up a full planet. I don't have either so I can't be sure but I am sure that those that do have them will chime in sooner or later. I, and a few others, use good ole Google Earth. We make up our own world then import it into G.E. There is a quasi-tutorial around here somewhere that shows exactly how.
It's in this thread:
Mixing hand drawn and randomly generated elements?
Just read through the posts and further down (page 2) the Master RobA explains it. As far as making up your own world to import into G.E., well there's a ton of tutorials here to help get ya started.
If this is pertaining to some of those artwork pieces from a few months back, then they used Photoshop and/or GIMP in a more painterly style. There are tutorials on the web for making them.