An openly licensed resource is any item that carries a license with conditions that grant permission for any one to make use of that work as long as they follow the conditions of the license. (Year of Open)
While you still need to confirm the images you're using carry an open license, the majority of the images/artifacts in this museum's digital repository have Creative Commons licenses.
This "living database" from the New York Public Library has new materials added every day and the majority of the items are openly licensed. The website does carry a warning about potentially harmful or difficult to view materials.
This digital collection includes rare books, journals, and manuscripts in addition to online exhibits. Items are marked for copyright, but the majority of the items displayed are in the public domain.
