var bg="http://i37.servimg.com/u/f37/10/07/34/13/bg-def10.jpg";

if(screen.width < 801) bg="http://i37.servimg.com/u/f37/10/07/34/13/bg-80010.jpg";
else if(screen.width < 1025) bg="http://i37.servimg.com/u/f37/10/07/34/13/bg-10210.jpg";
else if(screen.width < 1281) bg="http://i37.servimg.com/u/f37/10/07/34/13/bg-12810.jpg";
else if(screen.width < 1601) bg="http://i37.servimg.com/u/f37/10/07/34/13/bg-16010.jpg";

document.body.style.backgroundImage="url("+bg+")";