One of the fantastic resources I found for when putting together the new Free eBooks, Images, Music, and Videos guide is the Library of Congress Prints & Photographs Online Catalog. The Library of Congress has digitized many of their photo collections and organized them by topic for their users. Each collection and image contains information on the rights to the image (whether or not the image can be used, reproduced, etc without permission from the copyright holder) when known, information about the image, and more. This is a great resource for primary sources on a number of topics.
One of the larger collections the Library has digitized is their Civil War collection, including this image from the Battlefield of Gettysburg taken in July 1863-150 years ago!