I do not know the way your EPUB is constructed, but usually the first HTML file in the EPUB is something along the lines of "cover.xhtml" which only holds code to show the cover image.
If you add this code to your inline TOC, when clicked, will lead directly back to the "cover.xhtml" file:
Code:
<p><a href="../Text/cover.xhtml">Cover Image</a></p>
Red: Location of the file you want the link to send the reader to (Sigil places files in the Text folder, but your file placement may be different).
Blue: The text you want to display in the book.