Common Core ELA Standard 7 speaks to the importance of visual information: "Integrate and evaluate content presented in diverse media and formats, including visually and quantitatively, as well as in words." Historic photographs fit in here with nonfiction but also with fiction. Readers of historical fiction benefit from photographs of the era, especially if the novel isn't illustrated. Students who struggle with reading can have problems creating images in their minds of the story's setting including clothing, furniture, transportation, and so on. Photographs can add a whole layer of understanding and draw the reader deeper into the novel.
Pedagogic purposes aside, historic photographs are just plain fascinating. And they are so much more accessible than in the past. My favorite places to spend way too much time browsing are Flickr's Library of Congress photostream at flickr.com/photos/library_of_congress/ and New York Public Library's Digital Gallery at digitalgallery.nypl.org. The Flickr site has wonderful "albums" on curriculum topics like Abraham Lincoln (22 photos), "Best of Civil War Faces" (61 photos), "Women Striving Forward"--women's suffrage (34 photos) and WPA Posters (27). Besides the curated albums, you can also search by keyword. A search for "baseball" turned up hundreds of photographs; one for "Newport" produced dozens. A mouse-over gives brief information, while clicking on the photograph brings up much more as well as a larger image. You can move with arrows from one larger image to the next, too.
New York Public Library's Digital Gallery strikes me as a little harder to navigate but full of unexpected treasures. While reading the excellent YA historical novel Son of Fortune by Victoria McKernan (2013), I tried to envision the protagonist's visit to Peru in the 1860s. He has a license to transport in guano (which was a lucrative venture then). What did the guano mines look like? Since I had recently come across NYPL's Digital Gallery, I put in the search term "guano" and got back several right-on-target photographs of Peruvian guano mines in 1865; they look like hills of hardened salt. (Find the photos here and here.) I was also reading a novel about immigrants and found, not surprisingly, that a search on "Ellis Island" produces an abundance of photographs to browse. The more specific the search, the more specific the outcome. For example, searching "Susan B. Anthony" results in ten photographs of her plus a few other items.
When thinking about historic photographs, you might want to go back to Myra's November 11, 2012 post about a website with guidance for teachers on analyzing photographs (http://primarysourcenexus.org/2012/03/analyzing-primary-source-images-common-core-state-standards-alignment/) and her December 23, 2013, post on paying attention to whether features like photographs support the text or are simply add-ons.