Maybe your cover file is ".jpeg" instead of ".jpg"?
Does the cover appear in the EPUB if you use the "Insert Image" button in Sigil?
Are you using the latest version of Sigil (0.5.3)?
If you don't care about their SVG, I would just erase their code and replace it with something along these lines:
Code:
<body style="margin:0px;text-align:center;">
<div><img alt="image" src="../Images/cover.jpg" width="100%" style="max-width:530px; max-height:800px;" /></div>
</body>
The red section will make sure there are no margins on the cover page, and that the cover is centered horizontally on the screen.
The blue section will stretch the image to the edges of the viewing screen (keeps the correct ratio).
The green section is optional, and will make sure the image does not stretch beyond its actual resolution.