The British Library has provided online access to 1 million images which are in the public domain. These images are free to reuse and remix. Click here to see them.
"We have released over a million images onto Flickr Commons for anyone to use, remix and repurpose. These images were taken from the pages of 17th, 18th and 19th century books digitised by Microsoft who then generously gifted the scanned images to us, allowing us to release them back into the Public Domain. The images themselves cover a startling mix of subjects: There are maps, geological diagrams, beautiful illustrations, comical satire, illuminated and decorative letters, colourful illustrations, landscapes, wall-paintings and so much more that even we are not aware of."
Wikimedia Commons: Images is an excellent resource for finding and reusing text and graphics. The Wikimedia Foundation owns almost none of the content on Wikimedia sites - the content is owned, instead, by the individual creators of it. However, almost all content hosted on Wikimedia Commons may be freely reused (http://freedomdefined.org/) subject to certain restrictions (in many cases). You do not need to obtain a specific statement of permission from the licensor(s) of the content unless you wish to use the work under different terms than the license states, eg some licenses require that the original creator be attributed, etc. (See website for more information).
Some interesting examples:
Information graphics by subject: physics graphs, political charts, astronomical diagrams, acupuncture charts, eye charts, etc.
Drawings by theme eg allegorical drawings, drawings of buildings, drawings of death, drawings of plants, drawings of textile designs, etc.
Random: images with objects to indicate scale, round pictures, historical images, etc.