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CC0 1.0 Universal (CC0 1.0)
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the images can be modified and may be used freely for any application - also commercially and in printed format. 
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[2] https://creativecommons.org/publicdomain/zero/1.0/deed.en

https://pixabay.com/en/service/faq/
[1] https://pixabay.com/en/blog/posts/public-domain-images-what-is-allowed-and-what-is-4/