Pixabay and Unsplash etc are very useful, especially for creation of non-fiction books.
I see people saying that Pixabay images are CC0. However, that is not strictly true. Only content published prior to Jan 9, 2019 is CC0. Later content is not CC0.
It is:
"an irrevocable, worldwide, non-exclusive and royalty-free right to download, use, copy, modifiy or adapt the Content for Commercial or non-commercial purposes (Content License)…etc with “Prohibited Uses”–which are outlined.
Pixabay Terms of Service
I am proposing that rather than writing that as a custom license–since it’s almost the same as CC by 4.0–that you provide a “Pixabay license” with a link to the terms of service. Who knows, they might even make a deal with you for sending people their way to seek content?