Backup photos stored w/Photos app

If you are making regular backups with Time Machine, the Photos Libraries will be included in the Time Machine backup.

TimeMachine or cloning software as recommended by Matti Haveri will work well, as long as you are not using iCloud Photos Library. I would use both, Time Machine and clones. Are all your Photos Libraries local libraries, or are you using iCloud Photos for one of your Photos Libraries?

If you are using iCloud Photos and "Optimize Mac Storage" is enabled, neither a clone nor Time Machine will suffice as a backup, because not all original image files will be stored on your Mac. Then you will have to download the photos from iCloud to be able to create a backup. To backup the originals of an iCloud Photos Library use the command "File > Export > Export unmodified Originals" and save them on an external drive.

Copy the photo library to an external backup disk.

I periodically backup my 300 GB photo library folder to a disk image using Carbon Copy Cloner. Updating the backup usually takes just a few minutes. (I chose to backup to a disk image because transferring that single file is faster than copying the library itself because it contains tens of thousands of files).

p.s. I have my main image and video backup on plain folders on two identical external HDs. I use Photos as just a viewer and an aid to transfer albums to iOS devices. But I also consider those Photos libraries as a 3rd and 4th backup, with the 5th off-site backup being in my son's external HD.