then where are you seeing a problem?
Calibre Never removes something from the original on import.
The image in the LIT file is still there.
If you tick the option to :
Read the Metadata from contents, and there is a
Cover available. It will ALSO be shown in the Library view.
If you "Convert" a book, The conversion options will control wether Calibre ADDS the cover from the Library (view). Calibre will not remove an existing Cover image that Calibre did not ADD (the usual is create a "titlepage.xhtml" in your book).