I don't know how it works. Most of the photos are out of copyright based on age but they would have the rights to the particular copy of the image they have--just like I would on my copy of that same issue or postcard or cdv? They have some VERY common images--I can't imagine they hold the copyright on the image itself.
Mary Evans has images from ILN, The Graphic, The Sphere, The Sketch, The Bystander, The Tatler, The Illustrated Sporting and Dramatic News, The Illustrated War News and Britannia and Eve. If I own a particular photo that is from one of those magazines but also widely published elsewhere, and the image is more than 75 years old, I don't think that they can claim exclusivity. They can license them to people though but, couldn't I or someone else with a large royal image collection? Many of the recent photo books are put together at least in part from personal collections. Of course, Mary Evans can give you high-quality, high-res images.