<h20> could just go away. A containing element for that image isn't really necessary, the image does need to be moved into #container since that's what you want it positioned relative to.
You can still use that as an id on the image so you can specifically style it from your CSS. Like this -
Code:
<!doctype html>
<html>
<head>
<meta charset="utf-8">
<title>Untitled Document</title>
<style type="text/css">
#container {
background: none repeat scroll 0 0 #E6E6E6;
border-top-right-radius: 0;
box-shadow: 0 0 0 #C1BFBF;
margin-bottom: 0;
margin-top: 0;
position: relative;
}
.container_12 {
margin-left: auto;
margin-right: auto;
width: 960px;
}
img#h20 {
position: absolute;
left: 900px;
top: 10px;
}
</style>
</head>
<body>
<div id="container" class="container_12">
<img src='http://i46.tinypic.com/kbzv34.png' alt='none' width="304" height="470" id="h20">
</div>
</body>
</html>
align='right' is deprecated under your DocType and should not be used
style='max-width:100%' ...I think full width is a given