A backup outlives the phone. By default, deleting a photo on the phone deletes nothing on the server.
Mirroring
You can turn on mirroring for one device. Then a deletion on that phone moves the photo to the server's trash, never straight out. Only you empty the trash.
The trash
Items in the trash can be restored until you empty it. For a photo from a folder or drive on the server, emptying the trash forgets the photo and leaves the file where it is. See Folders and drives.