Everyone here is correct about having your photos backed up by doing a clone operation on Macintosh HD. However, I'm not sure that you want to clone the whole Macintosh HD as an automated backup task (for whatever reason). My reply would be pertinent only if you truly want a specific backup task for the the photo library alone.
Photos stores it's media in Users/Home/Pictures/photo library.photolibrary. You'd have to be asking Carbon Copy Cloner to backup that Pictures folder to get the recently-added photos in each successive backup.
Photo libraary.photolibrary is a somewhat unusual data structure, from the standpoint of a non-IT person like me. It looks like one gigantic file in Finder, but it may actually be a directory or folder to Carbon Copy Cloner. IF you have Carbon Copy Cloner's backup mounted as a drive, then you may be able to direct Photos to Carbon Copy Cloner's Photo Library.photolibrary using Photos/Preferences/General tab. I say "may" because this is certainly nothing I've tried to do. That would allow you to tell if the added photos are actually being backed up by the automated CCC incremental backup. Then you'd just direct Photos back to it's original location. If you find that your added photos are being backed up incrementally, simply note the size of that "file" in the CCC backup and periodically make sure it's incrementally growing in size as you accumulate more photos.
If you have the disk space on your external drive, however, the most reasonable thing to do is to set up an automated cloning task for the entire Macintosh HD because you probably need to be doing that, anyway.