var bg= "http://img836.imageshack.us/img836/1600/masde1600.png";

if(screen.width < 801) bg="http://oi56.tinypic.com/2n8zda9.jpg";
else if(screen.width < 1025) bg="http://oi54.tinypic.com/9k6l5i.jpg";
else if(screen.width < 1281) bg="http://oi54.tinypic.com/34jdv6g.jpg";
else if(screen.width < 1601) bg="http://oi51.tinypic.com/4sgpad.jpg";

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