The New York Public Library has gone wild and created a huge digital image gallery. You can use any of the images for free for educational/research purposes (see previous post on fair use). You can also pay a usage fee to use the works in print or on your website.
They have fantastic images ranging from “Empire and Regency: Decoration in the Age of Napoleon ” to “Pictures of Science: 700 Years of Scientific and Medical Illustration,” and many many more. You can browse the contents in a variety of ways including searching. Don’t miss this great resource.