Apple used to make you go out of your way to download an OS upgrade. Then, Apple started having those OS upgrade installers auto-download to the /Applications folder. Then Apple made it so OS upgrades appeared as regular updates.
In macOS 10.14.6, if you use dscl to delete the JPEGPhoto attribute, and then add in a Picture attribute, you’ll see one of the user pictures change, but the other one will default to the generic silhouette:
Unfortunately, you also can’t just script writing in the hex of the photo via a dscl . -create /Users/username JPEGPhoto ALLOFTHEHEXTEXT command. You will get a /usr/bin/dscl: Argument list too long error.