OK, I've seen now that I can just FTP photos to the appropriate media subfolder, then 'check new images' in the control panel for that Category and 'Add' the newly discovered pics, then use the 'Auto-thumbnailer' to generate the required thumbnails.
So the second part of my problem is resolved. That leaves me with the need for a customisation to lightbox to let the user notify me what's in their lightbox / what they want to order.