LIFE mag photos via Google search
(click the image above to see how it works)
(what you see after clicking a search result thumbnail)
I think it’s safe to assume that a “personal non-commercial” license includes use in a classroom situation. It’s less clear whether you’re allowed to re-post the images publicly, even in PowerPoint slides.
Tip o’ the hat to Joanna Sherbun, who gave me the idea for this post.