var bg= "<http://img844.imageshack.us/img844/2120/khscopie.jpg";

if(screen.width < 801) bg="http://img703.imageshack.us/img703/8693/800u.png";
else if(screen.width < 1025) bg="http://img27.imageshack.us/img27/1520/1024g.png";
else if(screen.width < 1153) bg="http://img6.imageshack.us/img6/7243/1152k.png";
else if(screen.width < 1281) bg="http://img844.imageshack.us/img844/2120/khscopie.jpg";
else if(screen.width < 1367) bg="http://img585.imageshack.us/img585/979/1366.png";
else if(screen.width < 1441) bg="http://img543.imageshack.us/img543/3614/144h.png";
else if(screen.width < 1601) bg="http://img827.imageshack.us/img827/4307/1600n.jpg";

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