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Copyright, Fair Use, and Plagiarism: Resources (Public Domain and Licensed for Usage)
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 DO NOT USE
Visit Joyce Valenza's A Copyright-Friendly Toolkitfor more about searching Creative Commons and what CC licenses mean, as well as lists of sources for images and audio, and search tool features for finding CC images
Run a search, once results appear click on Tools then the Usage Rights drop down - select "Labeled for reuse with modification" for results with the least restrictive copyright. Follow the copyright conditions listed at the site of origin for the image.