iCloud & iCloud Photos · Messages, Mail and device backups in iCloud
Does deleting an iCloud backup delete my photos?
Short answer: Not if iCloud Photos is on — the photo library is stored separately and isn't touched. If iCloud Photos is off on that device, its photos may be in the backup, so make sure they're saved elsewhere before deleting it.
Not if you use iCloud Photos — your library is stored separately and isn't part of the device backup, so deleting the backup leaves it untouched. If iCloud Photos is off on that iPhone, its photos may live only on the phone and in its backup, so deleting the backup removes that copy.
Before deleting, open Photos on the device or check iCloud Photos on your Mac to confirm your pictures are there. See deleting old backups.