The blog SKI-FFY posted scans of John Schoenherr's pictures for The Illustrated Dune. And he quotes the back cover copy...
"I can envision no more perfect visual representation of my Dune world than John Schoenherr's careful and accurate illustrations." - FRANK HERBERTHear, hear, Mr. Herbert!
As far as I know, Dad made the full-color paintings featured here for the 1978 Dune calendar, and then the publisher decided to use them in an illustrated paperback edition of the novel and asked Dad to flesh out the book with black and white illustrations. He made these using his dry brush technique where he would load a brush with india ink, wipe much of it off on a paper towel, then apply the brush to the rough textured watercolor paper. Once the basic composition was laid in, he would work out the finer details with a steel nib dip pen.