[please55]

Ahhh, thank you. Played around some more. Getting it finally.

Basically if its not declared in the manifest then calibre can't 'know' its there. It's not checking the image list for 'cover.xxx'; just the manifest directly.

Now just for tinkering purposes. I had calibre set up a cover page for a book. Then went in the editor, imported the same image as cover.jpg and simply changed all the references in the manifest/html from cover.jpeg to cover.jpg. (I also experimented with png separately.)

Then I clicked on Modify Epub again to do its thing. It seems that the command doesn't 'check' the epub. It appears to take the image set in the database and insert it into the epub file as a .jpeg regardless of what's there.

--

So one might want to be prudent about using 'Modify Epub' to update cover images if you're a stickler for that kind of stuff. It appears even if an epub does declare an image as a cover and has a corresponding html page for it - 'Modify Epub' is 'destructive' in the sense its not checking, it will overwrite it with its own version. Which 99% of people won't care about and I wish my mind would just let it go. But now that I get how it works, I do have the painstaking option of manually updating the manifest and importing the correct high rez files.