Here's a neat little collection of medieval illustrations. These feature people made up of geometric shapes and look great as a high-end print. You can use these images legally—they are in the public domain. “Public Domain” means that there are no known copyright restrictions on the work. This means that you are free to download, use, remix, edit and sell these images as part of your artwork or creative project.
The artist is Giovanni Battista Bracelli – Italian, circa 1624 – 1649.
I've got some examples at the end of the video on different ways you can modify the images for use as prints, posters or print on demand products (like if you are a designer on Etsy, Redbubble, Teepublic or Merch by Amazon).
(National Gallery of Art)