Photos (verified) exist of Jesse James, Wild Bill Hickock, Wyatt Earp (and brothers) and many more. Most of the "shootists" had their heyday in the decades following the Civil War (which ended in 1865)... especially the 1880's and photography was well established. I live in the western U.S., by the way and in a very short time can drive to the haunts of many of the well known.
You may have had an opportunity to see a Robert Redford movie a number of years ago entitiled "Jeremiah Johnson"). The movie was based (very loosely) on the life of Johnathan Johnston, known by the moniker of Liver Eating Johnston for his supposed habit of eating the livers of the Crow Indians whom he hated for some unspecified reason. Mr. Johnston was the sherrif for few years in Red Lodge, Montana, served previously in the Civil War and was buried in California in a War Veteran's cemetery but his body was moved to Cody, Wyoming due to the influence of Redford's movie. Even Johnston was photographed as seen here:
http://www.johnlivereatingjohnston.com/ ...