Try this - (for ePub 2, not sure about ePub 3)
- ensure you have the cover image elsewhere; e.g on the desktop
- remove the cover from the epub using the calibre Modify optional plugin
- open the the epub file in Sigil
- add the cover image to the Images folder (right click folder->Add existing files)
- with Tools->Add cover select the cover image file you just added to the Image folder
That should wrap the image in the required code in a 'cover.xhtml' file etc.
ADE is a better yardstick than calibre, because the underlying rendering code is closer to what many ebook readers use.
BR