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NASA photos are typically in the public domain, while some of the photos from other agencies claim to be governed by the "Attribution 2.0 Generic" license which requires giving appropriate credit to the agency that produced them. Make sure to check the license for each photo. Certain photos are provided by private citizens as a courtesy to a government agency. For example, this landscape is copyrighted by a private citizen, Natalie Bell, even though it is included the US Forest Service's photo gallery.
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