HOW THE ROCK WERE DONE:
:b2: Plane 3D for sky and Background
:b2: The DepthBrush
:b2: Pick a texture
:b2: Vary brush size
:b2: The apperance of the rocks used is directly dependant on the texture used.
Finally the texture I used was different bark textures imported into Zbrush.
The texture can be downloaded from the the link below.