BR
What I was implying (poorly

) , Was:
Calibre only inserts images that are part of the book or are managed by Calibre. Cover.jpg is the only image that Calibre
manages.
What I was also implying, was any additional 'Jacket' images were going to need to be hacked into the book directly.
The implication, is that Calibre should prevent a 'comment' mode that it is not willing to support (pass thru on conversion). What you see is what you will
see later

.