Hey gerryfromwilmington,
We see you're wanting to back up your iPhone, mostly what's in the Photos app on the Mac. Do you sync your photos to iCloud? If so, you can download them by following the steps here: Download iCloud photos and videos.
If the photos are on the Mac, you can move the Photos library to an external drive: Move your Photos library to save space on your Mac.
If Finder is used to back up, review this article on how to find the backup: Locate backups of your iPhone, iPad, and iPod touch. Keep in mind, if you need to restore the backup in the future, it would need to be moved back to its original location or it will not be detected.
Take care.