This is a 3″ x 6″ timber cut to a length of 6 feet, 10 inches, sanded and then stained with natural stain. It seems to float in the air because there are no visible shelf brackets. Holes were drilled into the back edge and then it was slid onto four large lag screws protruding from the wall studs.
One of the advantages of using a shelf for pictures (as opposed to mounting pictures individually onto walls) is that the pictures can easily be swapped out with other pictures without making any holes in the walls.
It really seems to float when viewed from this angle: