"Backup" means there's an original somewhere as well. Do you have them on your external boot drive?
If you're using iCloud Photos currently, the cloud is your primary storage, not a backup. If Photos > Preferences > iCloud > Download Originals to this Mac is selected, then you could consider the Photos library stored on your HD to be a "backup." If "Optimize Storage" is selected, you do not have locally-stored full-quality copies of all your images.
To withdraw from iCloud Photos you first need to be sure full-quality images are on your HD. They will already be there if you're set to "Download Originals to this Mac." Just turn off iCloud Photos.
If you're currently set to "Optimize Storage," switch to "Download Originals to this Mac" and then wait until Photos finishes updating. Then you can turn off iCloud Photos on the Mac.
That will leave you with one copy of your photos on your HD. To have a proper backup, you'll need to back that up with your favorite backup method (Time Machine, Carbon Copy Cloner, a cloud-based backup service...).
If you aren't already making a regular backup of the entire external boot drive, do it! External drives are more susceptible to file system errors than internal drives due to the fact that you're dependent on that thin, exposed external wire - if the wire became even momentarily disconnected during a write, you may end up with file damage.
Most folks wouldn't consider an internal drive as a "backup" as accessing that drive should its "enclosure" fail can be difficult. Externals are preferred as they can easily be moved to another computer.