This Open Culture article is from April, but sharing good news about public domain images is something that shouldn't be bound by time. (In other words, better late than never!)  Smithsonian Open Access provides now provides new platforms and tools, which give "easier access to more than 4.5 million 2D and 3D digital items from our collections—with many more to come. This includes images and data from across the Smithsonian’s 21 museums, nine research centers, libraries, archives, and the National Zoo." (Smithsonian) Note that you can search and retrieve only images that have CC0 licenses (free of copyright restrictions).

This is a collection that is worth bookmarking and using! Consider how you can connect images from the past to events that are happening today, which could help you broaden the media that you are using.