When juries specify that the artist must own
the copyright it normally means using your own image - rather than a copyright-free image taken by someone else.
Not all juries demand this, but that's my understanding of the rule.
Using copyright-free images (as from RIL) allows you to exhibit and sell work but normally credit should to be given to the copyright-owner (the photographer) when publishing details of the work.
Could a person ask the photographer for the "copyright" to the image on some kind of a basis?
You could, but I doubt the owner would sell it!
Granting permission to use the reference is a different matter
In UK law, the only time the copyright does not belong to the photographer is when the image has been commissioned as part of the photographer's job for example.