function positionBackground(){
//set this to the width of your background image
var backgroundImageWidth = 1600;

var size = getPageSize();
var body = document.body;
if( navigator.userAgent.indexOf('Safari') != -1 || navigator.appVersion.indexOf("MSIE") != -1)
{
 if(size.width % 2 == 1){
  size.width -= 1;
 }
}

body.style.backgroundPosition = size.width/2 - (backgroundImageWidth / 2);
}

function getPageSize() {
var myWidth = 0, myHeight = 0;
if( typeof( window.innerWidth ) == 'number' ) {
  //Non-IE
  myWidth = window.innerWidth;
  myHeight = window.innerHeight;
} else if( document.documentElement && ( document.documentElement.clientWidth || document.documentElement.clientHeight ) ) {
  //IE 6+ in 'standards compliant mode'
  myWidth = document.documentElement.clientWidth;
  myHeight = document.documentElement.clientHeight;
} else if( document.body && ( document.body.clientWidth || document.body.clientHeight ) ) {
  //IE 4 compatible
  myWidth = document.body.clientWidth;
  myHeight = document.body.clientHeight;
}
return {width:myWidth,height:myHeight};
}

window.onresize = positionBackground;
window.onload = positionBackground;