When we register a book for copyright, do we need to apply for
separate copyright registration for the photos in the book?
YES, they are separate
Text and photographs are separate applications (although each may use the 'standard application', if appropriate).
The USCO has information, sample applications, and video tutorials for the different types of works:
I could not find a FAQ or article that answers your question perfectly or I would provide a direct link (there may be one, but I stopped looking).
The photographs can be registered as a group, again assuming they have never been published. Previously published photos require a different application, and you cannot mix published/unpublished photos in the same group application.
There may be reasons to register the cover art separately. But if it uses one or more of the photographs, it makes sense to simply copyright those photos along with the rest. Each application costs money, and presumably the photos would be the important part of the cover art (not the typography and blurbs, which may not be yours). A second printing may re-work the cover art, and the photos used for the cover would still be protected.
If the book is already published, or about to be published, there is a short window (3 months) where the works can still be registered and be eligible for certain extra protections.