Photo Backup

What are you calling a backup? Backups only are backups if they are two separate copies on two different physical devices.

If you plan on removing the photos from your Mac and keeping them on a backup drive, that backup drive will also need to have the photos backed up to be "safe." No media is permanent.

Time Machine creates a backupdb folder on the drive dedicated for backing up on Time Machine, and manages that folder for future Time Machine backups. Anything kept separate on that drive will not be touched by Time Machine. Time Machine backups themselves will be erased if the original does not have the file in question after a certain amount of time. Time Machine keeps archives as long as there is space to keep hourly, daily, weekly, monthly, and annual backups. Any in between backups that don't have the original that you want handy will miss that data.

So you have one of two options:

1. Keep the photos you want on the original and use Time Machine to backup the original drive, and you'll always have a backup.
2. Keep Two separate external drives with the photos in their own folders if you don't have room to maintain the photos on your original drive.