<div id="div1"><img src="img.jpg" /></div>
div#div1{
position:fixed;
top:0;
left:0;
bottom:0;
right:0;
z-index:-1;
}
div#div1 > img {
height:100%;
100%;
border:0;
}
<div id="div1"><img src="img.jpg" /></div>
div#div1{
position:fixed;
top:0;
left:0;
bottom:0;
right:0;
z-index:-1;
}
div#div1 > img {
height:100%;
100%;
border:0;
}