Honestly i'm not 100% sure when it comes to adding information to existing entries, i usually just add to those manually, not sure if that functionality of this plugin is still working sadly, apologies for not being much help.
Even though the context menu only ever says "Add new images to..." i assure you it works as intended, i just added a new game just to check whether it had broken as i hadn't used it in a few days.
Just wondering if this plugin will be getting updated for Launchbox.Next? It works perfectly fine with the OG Launchbox layout but the next official release of Launchbox will only be using the new layout.