Use a Google Advanced Search to find images that are copyright free. On the image search page, click search tools and click the down arrow next to Usage Rights and select labeled for non-commercial reuse.
Unsplash: Free high-res photos to be used anyway. There are 10 new photos every 10 days.
Images can be used for any purpose without attribution.
Images can be used for any purpose without attribution.
Students and teachers can use for educational purposes without attribution.