The Biodiversity Heritage Library (BHL) has uploaded more than 150,000 images of biological sketches, some dating back to the 15th century, onto the internet. They’re all in the public domain, and free for anyone who wants them.

https://www.flickr.com/photos/biodivlibrary/albums

 

Just amazing artwork in some of the albums.

 

On a related note:
https://www.si.edu/openaccess

Welcome to Smithsonian Open Access, where you can download, share, and reuse millions of the Smithsonian’s images—right now, without asking. With new platforms and tools, you have easier access to nearly 3 million 2D and 3D digital items from our collections—with many more to come. This includes images and data from across the Smithsonian’s 19 museums, nine research centers, libraries, archives, and the National Zoo.