Michael, these are bump maps. Not sure if they were created in Blender or another package. I usually create objects in Blender, mapping surfaces to image files where needed, but then I use that image file to create SPEC and BUMP maps in Crazybump. Crazybump can also clean up the original photo image a bit if necessary. Even though the the image file is mapped to the object surface in Blender, I do not apply the SPEC and BUMP files until the object is imported into Terragen. Once I'm satisfied with how it looks in Terragen I usually save as a TGO.
AFA getting good with Blender, you don't have to be a Blender expert just to use the functions necessary for working with these rocks. Blender's learning curve is probably steeper than Terragen (unless you're working with blue nodes in which case being drunk, crazy or registered democrat probably helps) but there are many, many functions you won't be using. Kinda like using Microsoft Excel, where most people only ever use about 2% of the functions that are actually available).
What struck me about these demo rocks was that I don't think they meet the standards of 'Hero' objects. Think that's more a problem with the smaller sizes of the JPEG files. I'm going to reach out to Andrew Price and see if higher-resolution images are available. If so, I'll re-render and post here.