For some reason a perfect image size doesn't make the image show full screen, despite whatever image size is mentioned in the titlepage.xhtml file (big sigh). Even removing the size tags or putting them to 100% doesn't work, I always get the side margins. It's so ridiculous!
There's one body code that works for the titlepage.xhtml, I'm afraid I can't remember who gave it to me here on the forum (sorry). But I used that code and Sigil changes it to the code blow. If you put that in the titlepage.xhtml file all will be perfect. But, yes, lots of hassle. Kobo should just use the image it's given instead of do weird things with perfect images.
Replacing the images in kobo's image directory also sounds like a good idea, I've never tried that. But also sounds like a hassle as you have to keep track of which books exactly you have to put the cover in for... and do it again if you add new books...
Code:
<?xml version="1.0" encoding="utf-8" standalone="no"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN"
"http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<title></title>
<style type="text/css">
/*<![CDATA[*/
body.sgc-1 {margin: 0px; text-align: center;}
/*]]>*/
</style>
</head>
<body class="sgc-1">
<div><img alt="" src="../Images/cover.jpg" /></div>
</body>
</html>