Hi Doug,
Here's an example image to illustrate what I was referring to in the previous post.
The top section of the image is the rock object loaded in Lightwave. When first imported, the uv map is defined but there was no geometry assinged to it. This can be seen in the top quadrant where I've indicated "Before: no geometry assigned to uv map". For uv purposes, I split the rock down the center and laid it out as in the lower quadrant where I've indicated "After: geometry assigned to uv map". Then I resaved the object as an OBJ file.
I "photoshop'd" the petroglyph texture map so that it aligned with the uv layout, see the preview window in the Terragen portion of the example image. I assigned this texture map to the Colour image parameter and the Displacement image parameter of the Default shader, just as you had done.
Next I assigned Power fractal shaders to the "Colour function" parameter and "Displacement function" parameters.
When that was complete, I saved the rock object as a TGO, or Terragen object. I loaded the TGO as a popluation and scattered it around the ground so you can see how the rock instances still maintain their look.
Hopefully, this example shows that you can combine shaders togther to get the look you want, and the importance of having good uv coordinates on objects to start with.