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>