Oh I see ! I wasn't aware of these two modes triggered by this "secret" click. I actually don't think I ever noticed the center mark either, because it is hidden most of the time except when hovering over the corners (essentially putting it in a blind spot that the gaze is never focused on). Thank you for the explanatory video.

Why not simply put a button in the UI for this mode switch, next to Add/Remove/Align/Hide ? This would make it easily discoverable.

The question of how to snap to the corners of an already placed image object remains though. Is there any know workaround for that ?