One of my favorite sources of imagery is the Library of Congress. There are 1.2 million images in the Prints and Photographs Online Catalog. Many of the images have no known copyright restrictions, as you paid for them yourself with your taxes. I first went here to find images of family members from the Civil War and later. The Library of Congress had kindly created an entire sub-section for me of Civil War Generals. Along the way I even found other family members who weren't officers. Recently I was looking for imagery from World War II and found the 1930s and 1940s in Color set on flickr.com. There are many images that aren't necessarily thrilling, but are good documentation. And there are those hidden gems of beautiful photography.