Since I started using them earlier this year
Pixabay and Every Stock Photo have been my go-to resources for public domain images. This morning I learned from Tony Vincent that Bing has an option for finding public domain images. To access this setting go to Bing Images, enter your search term, then use the “license” menu to select public domain images. I’ve included a screenshot of the settings below.

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Applications for Education
Bing image search would probably be my third choice of places for students to search for public domain images after Pixabay and Every Stock Photo. It’s my third choice because, like Google Images, the “related” images and search suggestions might not always be appropriate for classrooms.