For clip art I use Nova Development’s Art Explosion 750,000 or 800,000 clipart collection. There is a lot of unusable clipart in these books, but the few that I use make it well worth the purchase price (~$99). There are other similar clipart libraries available but I cannot comment on their usability.
For photographs and illustrations I like Getty Images (www.gettyimages.com), iStockPhoto (www.istockphoto.com), Dreamstime (www.dreamstime.com), StockExpert (www.stockxpert.com), and BigStock Photo (www.bigstockphoto.com). When using Getty Images, be sure to choose only royalty-free images versus rights managed. It is far less expensive. For United States military photographs, visit the .mil websites. You may use the photographs in their image databases, but they request that you acknowledge where you found the photo and, when possible, acknowledge the photographer (i.e., "Photo courtesy of U.S. Army. Photo taken by Spc. D. A. Dickinson."). Be sure to verify that the image you are using is cleared for release and are considered in the public domain. This information is usually posted on the same page as their image library or on their “Privacy” or “Security” pages.