Then as I previously stated, the photos would not be included in a backup to iCloud.
See the following from What does iCloud back up? - Apple Support
Your iPhone, iPad, and iPod touch backup only include information and settings stored on your device. It doesn’t include the information already stored in iCloud, like Contacts, Calendars, Bookmarks, Mail, Notes, shared photos, iCloud Photo Library, My Photo Stream, and files you store in iCloud Drive.
1. If you turn on iCloud Photo Library on your device, your photos and videos are already stored in iCloud, so they aren't included in your iCloud backup.