Images on U.S. government websites are GENERALLY copyright free. Using Google Image Search, type your search term and then site:.gov, which will limit your results to government websites.
Example: Old Faithful site:.gov
There are some exceptions to this permission. See the How Copyright Applies to U.S. Government Works link below.
Additional sources of copyright friendly images:
How do I cite an image that I want to use in a publication, presentation, or website?
Attribution differs from a full citation. When attributing a work, you will simply give credit to the copyright holder.
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