The Robert Runyon collection is a real find. Thanks for passing it on.
There was another border photographer, W.D. Smithers, who was a fascinating character. He became a U.S. Army teamster on pack mule trains in 1915-1917, during the Mexican Revolution (Pershing, et. al.). He was already an accomplished photographer, and photographed cavalry, air, and transport avidly.
During the U.S. involvement in WWI, he taught aerial gunnery(!) in California. After discharge, he went back as a civilian/army muleteer on the border.
Book recommendation: "Chronicles of the Big Bend", Texas State Historical Association. Reprints many of his army photos, sketches of army mule tack, etc.
There are 9,000 photos by Smithers in the University of Texas at Austin's Harry Ransom Research Center. I don't think they're digitized yet.... I'll try to scan a few from the book soon.