/* css */
#bg {
	position:fixed;
	top:-50%;
	left:-50%;
	width:200%;
	height:200%;
}
#bg img {
	position:absolute;
	top:0;
	left:0;
	right:0;
	bottom:0;
	margin:auto;
	min-width:50%;
	min-height:50%;
}

/* html */
<div id="bg">
	<img src="images/background.jpg" alt="Background Image" />
</div>

Fullsize background image

css

CSS