The photograph included above comes from the Library of Congress, the George Grantham Bains collection. It shows a scene of corporal punishment (two men with hands and heads in stocks and another man below being whipped) at a prison camp in Delaware around the turn of the century. LC-USZ62-98905 (b&w film copy neg.).
I have found other pictures on line as well although none as compelling as the ones used in the documentary and its companion book.
One picture in the book of a collection of boys, perhaps eight to twelve, wearing stripes and chained together working in a field, is stamped "juvenile convicts at work in a field" and "copyright 1903 by Detroit Photographic Company," which turns out to have been a very successful photographic company that mass produced photos for popular consumption from the turn-of-the-century. The Library of Congress has a large archive of their photographs.
Also, another key topic that emerges in the film is D.W. Griffith's THE BIRTH OF A NATION (1915) as well as Thomas Dixon's play THE KLANMEN, which provided the basis for THE BIRTH.
I wrote an article in 2003 on THE BIRTH OF A NATION, which was published by THE CINEMA JOURNAL, 43, No 1, 2003.
The article provides much of the desirable background for race relations and could be helpful to read as a supplement to this documentary, which has some misleading characteristics I believe in that the analysis of images, music and cultural production by black or white artists isn't sophisticated enough. There isn't enough of a comparative grid of the various kinds of images that were available at the time. I, myself, had never seen that black "incubus" image before and am not sure what it means. In fact, there are no analysts of culture used as authoritative sources with the possible exception of Grace Hale, who is primarily a social historian, not a cultural historian I believe. I love her work but these people are using cultural production in a kind of shorthand to illustrate their overall assessment of a violent and fascistic time in American history. It is simplistic the way you would feed it to a child and yet because of the sadistic violence portrayed, it would be unsuitable for children.