Hmmm... currently the upload zone isn't restricted to a bounding box. The only way I can think get around this is to add a further layer that indicates to the user where they can place the upload zone.
Thank you for your reply. And while waiting for better days, I will do as you say: I will indicate in the Upload Zone (= photo) that it must be placed inside the Bounding Box ( = printable area). But in my case, it's not pretty to see this element wandering around anywhere in the canvas, even beyond the Printing Box.