One of the most important principles of copyright is "creative content". If an image has no creative content, then it can't be copyrighted. The digits of pi aren't copyrightable, because they contain only "information", not "creativity". Because of this principle, a completely black image, or a simple image of a triangle, is in the PD. In fact, typefaces aren't copyrightable in the U.S. (although they are just about everywhere else), so a picture of a word is also safe to use. But as you can imagine, there's a lot of gray area; that's what we have lawyers for. My advise is, don't push it. If a judge rules that an image has even a minute amount of creative content, it can be copyrighted, and different judges can have different standards.
But one of the pleasant off-shoots of this is that scans of paintings are generally not copyrightable: just the original artwork is. So if a painting was first published before 1923, then a scan of the painting is PD, even if the scan was made recently. A judge ruled in Bridgeman Art Library v. Corel Corp. that copies of artwork produce no new "creative content", so you're off the hook on that one. This opens a whole new array of PD images.
There are lots of great places on the web to find scans of PD paintings and drawings. Be careful though; some of them may be copyrighted if the original piece was first published after 1923, and it may be difficult to tell when a painting was officially published, so proceed with caution.
The three best sites I have found are the Web Gallery of Art, the Webmuseum of Paris, and the Art Renewal Center. The CGFA also has scans of artwork, and will have some pieces not listed in the links above. In addition, many museums, such as the National Gallery of London and the Louvre, sometimes have scans of their paintings available, but these esources are usually somewhat less useful.
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All information on this site is correct to the best I can determine; however, nothing on this page should be construed as legal advice, and I cannot be liable for any damages if this information is inadvertantly incorrect.