I got into an argument years ago with a cover designer who didn't want to use photos or portraits of kings and queens from the 19th century, saying that some museums claim they have the right to those images and/or to photos of them. When it's a 100-year-old or a 500-year-old painting, I tend to disagree, but IANAL. We didn't use people's portraits on the cover after all, yet I see them on other people's books constantly. And what about a photo of Queen Elizabeth II? Or a photo of the official portrait of QEII that's in the National Portrait Gallery in London? (The one with the cloak.)
They may have been thinking of Crown Copyright in the UK and other commonwealth countries. The Crown Copyright covers works that were created or published by or under the direction of the leading monarch. Under the 1988 revised Copyright Act, the Crown Copyright only lasts for 50 years from the date of publication or 125 years if the work is unpublished. But, there is a royal prerogative to issue Letters Patent to some works which can extend the Copyright term indefinitely. The
King James Bible is under a perpetual copyright because of that.
In the United States and many other countries, the
King James Bible is considered public domain. Within the United Kingdom, it is still protected though.
Whether photos and portraits of kings and queens are protected by in a similar manner, I don't know. I've never looked into that, but there's a possibility. Old images may be public domain in the United States and other parts of the world, but if they happen to still be protected in the U.K., then you might someday run into issues if you are selling books using such images.
IANAL but Crown Copyright and Letters Patent is something to be mindful of if using works related to U.K. royalty.