Did you go and read at the link. Your blanket statement is not accurate
My blanket statement is indeed accurate. The photographer who took the photographs in question did so with the permission of the owner of the slaves.
While slavery is now outlawed it was legal at the time and therefore the consent of the slave owner was required for the photographs.
The Constitution’s clause prohibiting ex-post facto laws prevents the photographer’s then-legal actions from being deemed illegal some 200 years after the fact.
Therefore the subjects (and their possible descendants) of the photographs have no valid claim on the photographs.