Ok...I'm not sure but, that mesh you are using looks like a Hankster rock formation..right?
If so, the mesh has "issues". I noticed when I imported some of them into Zbrush that they have tiny protrutions jutting out of the mesh in some places. Which can be repaired in zbrush. But as Agiel said.....there is an easier way..photoshop.

It's a shame stupidity isn't painful.