You can simply add an img src Attribute with id:
<body>
<img id="backgroundimage" src="bgimage.jpg" border="0" alt="">
</body>
and in your CSS file (stretch background):
#backgroundimage
{
height: auto;
left: 0;
margin: 0;
min-height: 100%;
min-width: 674px;
padding: 0;
position: fixed;
top: 0;
width: 100%;
z-index: -1;
}