Create an html file containing the following and import it into your project.
Code:
<?xml version="1.0" encoding="utf-8" ?>
<!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>
<meta http-equiv="Content-Type" content="application/xhtml+xml; charset=utf-8" />
<link rel="stylesheet" type="text/css" href="style.css" />
<title></title>
</head>
<body>
<div class="centerAligned">
<img class="imgstyle" alt="" src="[path to your image file]" />
</div>
</body>
</html>
Edit your css to add class selectors with the styling you want, e.g. 'centerAligned' is usually just {text-align: center} and 'imgstyle' will be something like {height: 100%}, though obviously you can alter these as needed and the selector names are arbitrary.
Move the new imported file up or down until it's in the right place, then compile your book.