In Sigil, the file already has cover semantics set, but after unchecking and then rechecking, with a very small change ocurring right at the top of the OPF, then the cover change seems to take, even without the tweak, Calibre did show the cover image (a generic placeholder like the one shown in this writeup
http://www.howtogeek.com/73979/how-t...-with-calibre/), such as it was - example
http://www.gutenberg.org/ebooks/6331 - not the best example, as available covers are rather generic too.