How can I delete a photo on multiple devices without having to delete it manually everywhere? I'm using Photos for iOS and MacOS to store my photos. When deleting a photo from either device, nothing happens with the photos on the other, not even after syncing. How can I avoid having to manually delete a photo on both devices for it to disappear? I'm not using iCloud sync.
Without iCloud, the images are just imported, not synced. There's no tangible connection between the two. This the same behavior has you'd find with a camera's memory card.

Before iCloud, images were synced via iTunes. You can still sync images back to the iOS device via iTunes, but if you do that the iOS device can't delete them.