Old Maps -- Old maps online portal: http://www.oldmapsonline.org/. Originals of the documents are in the public domain. Cartography Associates claims a copyright on the digital versions, which are more than likely watermarked. They do freely share the digital copies under a Creative Commons NC-SA license.
National Gallery of Art images -- https://images.nga.gov/en/page/show_home_page.html. Originals of the art works are in the public domain. The National Gallery does not claim copyright on the digital images and allows use for any commercial or non-commercial purpose.
School Clip Art -- http://www.school-clip-art.com/. Clip art, all cartoonish, that is categorized into groups that youngish students will enjoy. All images are owned by the website owner, and are licensed out for use in worksheets, web pages, etc. but only with a link-back and citation. Caution: each category has a few "free" items, but with a convenient link to "more _____ images" that are NOT free but can still be saved with a right click. This is a copyright infringement mine field.
FreeFoto.com -- http://www.freefoto.com/. Photos that are free to use for any online purposes if used with attribution and a link-back. For non-commercial purposes, a Creative Commons non-commercial, no derivatives, attribution license allows various uses as long as no sales and attribution.