var bg= "http://picload.org/image/ddraaop/backgroundforum1.jpg";

if(screen.width < 641) bg="http://picload.org/image/daddgag/backgroundforum6.jpg";
else if(screen.width < 801) bg="http://picload.org/image/ddrdigd/backgroundforum8.jpg";
else if(screen.width < 1025) bg="http://picload.org/image/ddiwwcc/backgroundforum1.jpg";
else if(screen.width < 1153) bg="http://img1.picload.org/image/ddrdigc/backgroundforum1.jpg";
else if(screen.width < 1281) bg="http://img1.picload.org/image/ddrdwdp/backgroundforum1.jpg";
else if(screen.width < 1369) bg="http://picload.org/image/ddiwwli/backgroundforum1.jpg";
else if(screen.width < 1401) bg="http://img1.picload.org/image/ddrdwad/backgroundforum1.jpg";
else if(screen.width < 1441) bg="http://picload.org/image/ddraaod/backgroundforum1.jpg";
else if(screen.width < 1601) bg="http://picload.org/image/ddiwwrc/backgroundforum1.jpg";
else if(screen.width < 1861) bg="http://picload.org/image/ddraaoc/backgroundforum1.jpg";
else if(screen.width < 1921) bg="http://picload.org/image/ddraaop/backgroundforum1.jpg";
else if(screen.width < 1921) bg="http://picload.org/image/ddraalg/backgroundforum1.jpg";
else if(screen.width < 2049) bg="http://picload.org/image/dadrwwd/backgroundforum2.jpg";
else if(screen.width < 2561) bg="http://picload.org/image/daddggi/backgroundforum2.jpg";
else if(screen.width < 3201) bg="http://picload.org/image/daddgri/backgroundforum3.jpg";
else if(screen.width < 3841) bg="http://picload.org/image/daddgll/backgroundforum3.jpg";
document.body.style.backgroundImage="url("+bg+")";