This is an example that I always use. It is always centered...
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>
</head>
<body>
<div>
<svg xmlns="http://www.w3.org/2000/svg" height="100%" preserveAspectRatio="xMidYMid meet" version="1.1" viewBox="0 0 573 800" width="100%" xmlns:xlink="http://www.w3.org/1999/xlink">
<image height="800" width="573" xlink:href="../Images/cover.jpg"></image>
</svg>
</div>
</body>
</html>
The red parts must be changed to the correct values. Please take care, a lot of things are case sensitive.