I don't believe you can do what you want in just CSS. You need some html code also.
Is this close to what you want to do: http://houstonlively.com/test/bgimage.html
The page has a background image that adjusts to the size of the browser window.... the caveat being that the image is also distorted when the window is not the same aspect ratio as the picture.
Anyway, here's the page code for the above url:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<title>Background to fit screen</title>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
<meta http-equiv="Imagetoolbar" content="no">
<style type="text/css">
html {height:100%;}
body {height:100%; margin:0; padding:0;}
#bg {position:fixed; top:0; left:0; width:100%; height:100%;}
#content {position:relative; z-index:1;}
</style>
<!--[if IE 6]>
<style type="text/css">
html {overflow-y:hidden;}
body {overflow-y:auto;}
#bg {position:absolute; z-index:-1;}
#content {position:static;}
</style>
<![endif]-->
</head>
<body>
<div id="bg"><img src="bgimage.jpg" width="100%" height="100%" alt=""></div>
<div id="content"><p><b><font color=yellow size=7>Place Page Content Here</font></b></p></div>
</body>
</html>
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