Before I replace the existing cover.jpeg I save it to the desktop -- right click cover in Book Details select Save cover to disk -- it will save as
{title}.jpeg. I open the jpeg in Irfanview and save it as a jng flle. Then I drop the jng file into the Book Details pane, calibre doesn't know about JNG files so it regards it as a format file.
I delete the jpeg and jng file from the desktop and then set about finding and uploading a new cover from the 'net.
I've never wanted to have an alternative cover in a book format file - e.g. the ePub. If I did, I'd have to do what retired biker/the ducks do with an ePub Editor which probably means I'd have to convert the JNG back to JPG.
If I have more than one alternative cover (or any other images) I want to keep for a book, I put them in a multi-image TIF and drop that on the book details panel. This requires the following tweak:
BR