1 Go to the overrides box and add the following code in between the GLOBAL_HEAD tags:
<style type="text/css">

<!--
body {
margin-top: 250px !important;
background-color: #99CCFF !important;
background-image: url(URL OF YOUR HEADER) !important;
background-attachment: scroll !important;
background-repeat: no-repeat !important;
background-position: top !important;)

-->
</style>
2 Margin-top is how far from the top of the page you want the image to appear in pixels. Everything else should be self-explainable.