Most contributors to C/Stock Images do not request payment back for this. For the @Calendars project, those photos are selected where the artist does not charge for them. I provide a link to the post and the author of the photo. I think that the author's credit percentage is a very good practice. But I need to automate it, manually it is very time consuming.
The benefit is to extend the life of the photo. As far as I know, old unreadable photos are deleted after a long time of not being used by the system to free up disk space. I manually read the photos, then I extend their life.
Thank you for your inquiry and for sharing the Friday calendar page! 😉