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Open Educational Resources: Open Media
A guide for finding openly licensed textbooks, publicly-available databases, and other educational resources
the DPLA brings together digital collections produced by partnering institutions across the country. Images are available on the open web, but may be subject to reuse restrictions
Astronomical pictures of Messier deep space objects (nebulae, clusters, galaxies, etc.). Most of these are public domain and intended for private use. Copyrighted photos have a tag on the picture.
You can also find resources licensed for reuse via Google Images. Select "Tools" and then "Usage rights" to filter results based on machine-readable creative commons licenses.