Want an amazing source of free images? Try the US government…
Photograph of a Female Demonstrator Offering a Flower to a Military Police Officer: 10/21/1967

You can find great photos, propaganda posters, declassified top secret documents, and more using the Archival Research Catalog. Just be sure to select “Digital Copies” when you search to avoid being disappointed. Many of the images and videos (yes, videos) in the catalog are not yet digitized.

Archival Research Catalog of the National Archives