Unsplash - Beautiful photos licensed for reuse without attribution
Pixabay - "All contents are released under the Pixabay License, which makes them safe to use without asking for permission or giving credit to the artist - even for commercial purposes."
Photos for Class - "All photos shown are, to the best of our (and Flickr's / Pixabay's) knowledge, licensed by Creative Commons for public use."
Wikimedia Commons - "Wikimedia Commons is an online repository of free-use images, sounds, other media, and JSON files. It is a project of the Wikimedia Foundation."
Pics4Learning - "Pics4Learning is a curated image library that is safe and free for education. Teachers and students can use the copyright-friendly photos and illustrations for classroom projects, web sites, videos, portfolios, or any other projects in an educational setting."