SpaceX media no longer lists its images as being in the public domain. Even on Flickr, it uses a CC-BY-NC license. However, old images (from here to here) had indeed been placed into the public domain and have no copyright restriction (including some images on Flickr)

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Rijksstudio

Rijksmuseum digitised and made available public domain works from several artists including Rembrandt, Vermeer, and others

New images on Pixabay are not in the public domain, as they are released under the Pixabay License (among other things, incompatible with the GPL). Pixabay switched to the new license in 2019. Prior to that change, Pixabay used the CC0 waiver. Therefore, old images are still in the public domain, despite being misleadingly tagged with the Pixabay License. Before using an image, check what the state of the terms of service was at that point in time

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