The New York Public Library website has made available a very large number of public domain images of items in its collection. Although this is a US site, its collection embraces items relating to people, places etc outside the USA. The images include documents such as letters as well as photos and book illustrations and the like. I have now added the site to the free space page listing galleries and collections which have given permission for the use of images.
Please note that not all digitised images on the site are public domain. The search engine enables you to confine your search to public domain items.
Thank you to WikiTree member Susie (Potter) Officer for drawing my attention to this site. If anyone knows of other sites which can usefully be added to the free space page, please let me know.