Today, Google has announced changes to how Google Drive and Photos work together, including the end of automatic syncing of photos between the two services. Google says it has received feedback that the connection between the two platforms is confusing, and it's attempting to clear things up.
This means that starting next month, photos and videos you add to Google Drive won't be visible in Photos, and vice versa. Likewise, if you delete items from either of the platforms, they won't automatically be deleted from the other. Files that have been previously been synced across the two services will remain in both, however, so you don't lose anything you already have.
Going forward, should you want to add photos from Google Drive to Photos, there will be a new "Upload from Drive" option in the web experience for Photos. This will let you choose from any image or video files in your Drive, including items shared with you, and you can then have them added to the Photos service. However, since syncing is no linger enabled across the two services, items you copy in original quality from Drive to Photos will count towards your quota in both services, unless you choose to delete them from either side.
If you're using the Backup & Sync app on desktop devices, it will continue to work, and high-quality backups won't count towards your quota. Original quality files that are backed up to both services using the app will only be counted once, so that remains the same.