Even if you download the Cover with calibre, if it is not there in the actual ePub, it will not be displayed until you actually convert them. The jpg in the folder is just there for calibre to display, you don't need it at all. The Kobo will use the cover that is in the book. You don't need to copy the jpg file in it. When the KT processes the file, it will generate a cover from the one that is in the epub.
ePubFixer - http://code.google.com/p/epubfixer/
Edit your TOC, by Adding, Renaming, Shifting Chapter, Change chapters order or nest chapter into Part or Book easily by Drag and Drop. Split files by Chapters & Auto Detect phrase for your chapter heading. Also edit the reading order, fix margins & Edit your file with Sigil and restoring your previous TOC that was lost. Edit your cover without reconverting the file, Create an Inline TOC