Recently, the New York Public Library released more than 180.000 images for free downloads and unrestricted use to their digital collections site. The scans are high quality, the files have fairly hiRes versions (tiffs) and the collection is well sorted via tons of browsable key words.
The announcement: http://www.nypl.org/blog/2016/01/05/share-public-domain-collections
And here is how to get to them:
Use the search and make sure to filter for public domain only here:[LINK] If you don’t check that box, you might end up searching for copyrighted images, so please be thorough.