That sounds much more complicated than it should be to achieve what you want!
You should be able to simply
- add the fb2 book to Calibre
- set the cover in the Calibre metadata
- convert the book to epub
It is important you set up the cover and metadata before doing the conversion if you want them to be embedded in the epub.
There will also be a cover.jpg file in the ebook folder, but this will be in addition to one embedded in the ebook. Calibre always keeps an external copy of any cover image as not all formats support embedding the image.