The ones I know well, for free use that I have used are:
https://www.pexels.com/submit-photos/
https://pixabay.com/en/
https://www.deviantart.com/resources/
Although I have heard that there are also sites, where they can be sold in the same way. I do not know much about it, but I guess it's according to the tastes of the author.
thank you very much for the info my friend :)