There are some great high res photos in these image databases.
High res selections from Library of congress:
The National Archives on flickr:
https://www.flickr.com/photos/usnationalarchives
Library of Congress:
New York Public Library:
http://digitalcollections.nypl.org/
The public domain review:
http://publicdomainreview.org/collections/
The Met Museum of Art: Image Resources
http://www.metmuseum.org/about-the-met/policies-and-documents/image-resources