The Metropolitan Museum of Art has recently placed over 200,000 images of the collection in the public domain.
Images designated as “Public Domain” can be used — at no charge and with no paperwork — as you wish. Images can be used for both commercial and non-commercial purposes.
In poking through the collection, it’s several lifetime’s worth of work. Some person (most likely persons) has posed and captured each of the items in the Met’s catalog, whether the item is currently on display or not.
More information an a link to the collection can be found on the Met’s web site: http://www.metmuseum.org/about-the-met/policies-and-documents/image-resources
And, my own irreverent use of this new policy!