Pixabay is a vibrant community of creatives, sharing copyright free images and videos. All contents are released under Creative Commons CC0, which makes them safe to use without asking for permission or giving credit to the artist - even for commercial purposes.
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All images and videos on Pixabay are released under the Creative Commons CC0. Thus, they may be used freely for almost any purpose - even commercially and in printed format. Attribution in appreciated, but not required.
Identifiable people may not appear in a bad light or in a way that they may find offensive, unless they give their consent.
Do not suggest endorsement of products, services, etc. by depicted people or organizations. For example: do not use an image of NASA and place it adjacent to your own product in a way that suggests NASA would recommend the product.
In addition to these common sense regulations, there is more to know:
Model- and Property Release