Many homeschoolers like to do art studies and with means perhaps acquiring prints to use for those studies. Many copyright conscious homeschoolers will not just go to google to print out images. They don't do that because they want to honor copyrights. Plus many are also bloggers and look for ways to acquire images for their blogs.
Well, today I learned that the Art Institute of Chicago has released over 50,000 images of many artworks and pieces to the public domain and have assigned them the CC0 public domain designation. This designation means those images can be used for commercial and non-commercial reasons for free with no additional permission from the museum. They do have some rules to follow, but if you are familiar with creative commons it seems to be the typical rules.
Images such as these:
