New York Public Library - public domain
On 6 January 2016, The New York Public Library made over 187K digital items in the public domain available for high resolution download.
The images can be viewed at the link below. You can also reach them via this link (http://publicdomain.nypl.org/pd-visualization/) where they are displayed in pd visualisation and can be grouped according to collection, genre, color or the century in which they where created.
Each image is stored with a richness of data about it, simply accessible by clicking on the image itself.