http://www.john-leech-archive.org.uk/ John Leech sketch archives 1841 until 1864 from Punch magazine The page states they are long out of copyright and that they are believed to be public domain.
http://beinecke.library.yale.edu/dl_crosscollex/ Yale University's Beinecke Rare Book & Manuscript Library's online collection of digital images. Most will be {PD-art}. 90,000 images from rare books and manuscripts, search by keyword.
http://digitalgallery.nypl.org NewYork Public Library. Over 370,000 images scanned from books. Including illuminated manuscripts, historical maps, vintage posters, rare prints and photographs, illustrated books, printed ephemera, and more. There is a per-image usage fee even for public domain images.
Photos of the Great War many images of World War I, scanned in from public domain resources. Slight usage notice, which is probably compatible with GFDL (requires attribution and link, as does GFDL, requests e-mail notice of usage).
The Heritage of the Great War has several colour pictures from World War I (likely all PD, since the effort taken to scan/reproduce them probably fails to qualify for being a derivative work).
http://www.loc.gov/rr/print Portraits of famous people, Library of Congress (not all public domain; remember, anything prior to 1923 is, and post-1922 images on the site that are copyright free are indicated)
http://www.state.gov/ U.S. Department of State - good for pictures of politicians and diplomats, but check that images do not have an alternative copyright. If a copyright is indicated on a photo, graphic, or other material, permission to copy these materials must be obtained from the original source. For photos without captions or with only partial captions, hold your cursor over the photo to view the "alt tag" for any copyright information.
