Digital Library Image Collections
I've been looking around for sites with prints and photographs NML could use and found these sites at major libraries.
Wikimedia Commons is a media file repository making available public domain and freely-licensed educational media content (images, sound and video clips) to all. It acts as a common repository for the various projects of the Wikimedia Foundation, but you do not need to belong to one of those projects to use media hosted here. The repository is created and maintained not by paid-for artists but by volunteers. Wikimedia Commons uses the same wiki-technology as Wikipedia and everyone can edit it. Wikimedia Commons currently contains 3,394,595 files and 84,240 media collections. Unlike traditional media repositories, Wikimedia Commons is free. Everyone is allowed to copy, use and modify any files here freely as long as the source and the authors are credited and as long as users release their copies/improvements under the same freedom to others. The Wikimedia Commons database itself and the texts in it are licensed under the GNU Free Documentation License. The license conditions of each individual media file can be found on their description pages.
- Library of Congress Digital Prints & Photographs Online collection. (Search the collection now.) The good news here is that they have done the rights search for these items and explicitly tell you whether there are any restrictions on copyright, publication, etc. They also have various sized images, from smaller images suitable for use on a web site up to tens of megabytes in archival size such as would be suitable for creating posters by a commercial printer. The search site is keyword searchable within the bibliographic entries of the images.
- New York Public Library Digital Gallery provides access to their digital photo collection. They don't seem to have poster-sized replicas, but do seem to have more coverage than the Library of Congress collection for things like illustrations from books in the public domain. From what I can tell, all the photos are available as public domain digital images.