Here is a link to the U.S. National Archives. You can explore within the site from that page to find some of the many copyright-free photos owned by the government. I'll do a little digging soon to find some of the Ansel Adams photos - there are a number of sources for them, and for landscape painters many of them will be of interest. (Adams and Edward Weston developed processes for heightening tonal contrasts in black and white photography - probably why so many of his images are so striking: think "values"...)
National Archives Online Exhibits Page: http://www.archives.gov/exhibits/index.html
Here's a quote from the National Archives FAQ page about using the images in the archives (taken from: http://www.archives.gov/faqs/
May I reproduce images from your web site?
The vast majority of the digital images in the Archival Research Catalog (ARC) are in the public domain. Therefore, no written permission is required to use them. We would appreciate your crediting the National Archives and Records Administration as the original source. For the few images that remain copyrighted, please read the instructions noted in the "Access Restrictions" field of each ARC record.
Please note that a few images on other areas of our web site have been obtained from other organizations and that these are always credited. Permission to use these photographs should be obtained directly from those organizations.
I really recommend browsing the Archival Research Catalog. There are some incredible photos there. Some? Try: lots of photos.
Go to: http://www.archives.gov/research/arc/topics/
to start your browsing. It's also a great way to learn some U.S. history, if you're interested.
If anyone knows of links to British or European or any other country's public domain archives, I'd love to see it.
I'll update links with photos and describe them more particularly as I find them.