I recently came across the J. Paul Getty Museum, as an online source for public domain materials. The site is available here. As per rule of thumb, I made sure to check their site’s rights statement.  The site provides access to the collections of the Getty Research Institute. The collections include digital images of photographs and oral histories. These are mostly related to art history. Getty also grants access to its virtual libraries for books , publications and research collections. The J. Paul Getty Museum also provides teaching resources such as lesson plans, curriculum resources and activities for students.