Dover's collection of clip art and design images on CD-ROM offer hundreds of
permission-free images for your use on a wide range of subjects. Decorative
patterns, Victorian designs, Civil War illustrations, and much more. All for
half the price of a single piece of clip art found elsewhere!
http://store.doverpublications.com/pa0304.html

Also, if you like old books as I do, be sure to visit www.fromoldbooks.org . This site contains scanned images, engravings and pictures from antiquarian books (b&w and color), as well as photos of the books themselves. Each image is linked from an HTML page that tells you the copyright status of the image: public domain; permission required; non-commercial use only, etc. You will find lots of English castles, old churches, country cottages, the Geneva Bible, Sir John Tenniel's illustrations from "Alice in Wonderland," colorful illustrations by Arthur Rackham (the famous children's book illustrator), Medieval and Gothic images, Elizabethan art, Roman architecture, illustrations from "Torments and Tortures of the Christian Martyrs," page details from the illuminated "Book of Hours," ornamental alphabets, etc. Even if you don't need any pictures, this site is interesting just to look at!