Ok, first, you need to use CSS to define the background. Here's an example of a very simple header:
<table width="100%" cellpadding="0" cellspacing="0" class="header">
<tr><td><img src="{$ImagesDir}/spacer.gif" width="50" height="100" alt=""/></td></tr></table>
Then, in your skin1.css file, you place this:
.header {background: url(images/banner-01.gif); background-repeat: no-repeat;}
Replace the sizes in the spacer.gif to correspond to the height of your banner.