Well they're either linked to your Google+ account or your Backup Assistant account, even if you didn't sign up for it. Mine were on BA, so I had to log into the website on my computer and delete them in there. Only took a few minutes, still kind of a hassle though...
I think you have to go into your backup assistance+ settings in your phone and unchecked pictures, if you haven't done So. Are it will just continously back them.