Z-index can be used to place an element "behind" another element.

Code:
<html>
<head>
<style type="text/css">
img.x
{
position:absolute;
left:0px;
top:0px;
z-index:-1
}
</style>
</head>

<body>
<h1>This is a Heading</h1>
<img class="x" src="bulbon.gif" width="100" height="180"> 
<p>Default z-index is 0. Z-index -1 has lower priority.</p>
</body>

</html>