Link: A Clipart History - Public Domain Clipart

Free high-quality public-domain clipart covering all ages of the Worlds History from pre-historic to now. You may use these images for any purpose.
Most images are black and white but the site could be useful when working on History projects? [Eliminate results that other methods of image search may throw up]

One of the top results when using the search term: henge:

Some images are accompanied by text to give some background information for the image displayed.