The New York Public Library has released 180,000 public domain images, available for free and unrestricted use. The images may be downloaded as high-resolution files. The NYPL has also worked to improve the interface to their collections, making it easier to search, filter, and download the public domain content. Search their collection here: http://digitalcollections.nypl.org/. They have also introducted a new visualization tool to explore the public domain collection: http://publicdomain.nypl.org/pd-visualization/.  More technically inclined users will also be excited to learn that the NYPL has a collection API, enabling bulk use and analysis of their images and data. Learn more about their public domain collections on their website: http://www.nypl.org/research/collections/digital-collections/public-domain