Copyright law (title 17 of the United States Code) in the U.S. is supposed to protect a creator's rights to their work while also encouraging innovation. This page is designed to focus on that last part and your right to use/copy some material (images, etc.) for educational purposes under fair use law. That said, you are explicitly working in an academic environment, which means you need to cite your sources and give credit to the material you are drawing on.
When you are working in an educational environment, you have a lot of lee-way to use material with proper attribution. That said, when you are working on a public-facing (e.g., a website) project, you will be expected to adhere to higher standards of compliance than for an internal project (e.g., a paper submitted on Canvas). To be completely use you legally use material, consider the following:
The answers to each of those questions should give you insight into whether it's fair to use. For example,
There are other places around the web to find material that is clearly under free use. That said, with many sites you need to check individual images, etc., for their licenses. Here are some places to look that often have material you can use. Still, make sure to check.
Attribution is about ascribing a work to a particular creator/writer/artist, etc. You, as a historian, should follow Chicago rules for citing to artworks and more in your notes and bibliographies. But, within a caption (under an image on a WordPress page, for example), you should also consider the following statements:
For artworks, from photographs to paintings and sculpture, you should include the artist, title, date of creation/completion, and then information about the medium (what is it and how big) as well as location (such as physical Library or Archive). If you consulted the work online, include a clean URL (CMS 14.235).
N: 7 Cort Cornelis, The Battle of Zama, 1600-1790, engraving, 43.7 x 58 cm, Library of Congress, https://www.loc.gov/item/2012645609/.
B: Cornelis, Cort. The Battle of Zama. 1600-1799. Engraving, 43.7 x 58 cm. Library of Congress. https://www.loc.gov/item/2012645609/.
Note: After the creator's name, you can include a note of what kind of artist they are (e.g., sculptor, photographer, etc.).