Alternativ könnte auch das hier sein, was du suchst
html, body{
width: 100%;
min-height:100%;}
body, html {
height:100%;
}
#header {
}
.bgimage{
height: auto;
left: 0;
min-height: 100%;
min-width: 1024px;
top: 30px;
width: 100%;
}
#box1 {background:#ff0000;height:400px;}
#box2 {background:#c0c0c0;height:400px;}
---------------------------------------------------
<div id="header">
<img src="http://magazin.salzburgerland.com/wp-content/uploads/2012/08/hochkonig-007.jpg" class="bgimage" />
</div>
<div id="box1">a
</div>
<div id="box2">b
</div>