Here are some things I already tried.
1) deleting the file off of Kobo and then reloading it. Just in case it was storing an old version that I had looked at during the editing process.
2) deleting and re-adding the image in Sigil.
3) One former post in these forums mentioned that the name must be cover.jpg, but none of my other cover images are named that and they display just fine.
Here is the code for my cover page-this was created by Sigil and I compared it to a cover page that is working fine and I don't see an issue.
<?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>Cover</title>
</head>
<body>
<div style="text-align: center; padding: 0pt; margin: 0pt;">
<svg xmlns="http://www.w3.org/2000/svg" height="100%" preserveAspectRatio="xMidYMid meet" version="1.1" viewBox="0 0 1040 1477" width="100%" xmlns:xlink="http://www.w3.org/1999/xlink">
<image height="1477" width="1040" xlink:href="../Images/Misericordia%20Readers%20Second%20Cover%20copy.jpg "></image>
</svg>
</div>
</body>
</html>