Flickr, the photo-sharing website, has a section they call The Commons. The Commons began in January of 2008 when The Library of Congress released about 1500 photographs from their collection to display on Flickr. Since then, The Library of Congress Flickr collection has grown to almost 9000 photographs. Several other libraries and museums have added photographs from their collections. If you search The Commons any photographs you find will be completely free of copyright restrictions. If you follow a link to the museum’s full collection on Flickr, leaving The Commons, some of those photographs may have copyright restrictions. The description of the photograph should indicate whether or not there are any restrictions.
[Note: The 1860s image of the Jefferson Barracks military post just South of St. Louis, Missouri came from the New York Public Library collection at The Commons.]
For those seeking Civil War photos for Memorial Day - A search for the words "Civil War" produces over 1100 results.