var bg= "http://www.oubien.com/furious/web/bg_1920.jpg";

if(screen.width < 801) bg="http://www.oubien.com/furious/web/bg_800.jpg";
else if(screen.width < 1025) bg="http://www.oubien.com/furious/web/bg_1024.jpg";
else if(screen.width < 1281) bg="http://www.oubien.com/furious/web/bg_1280.jpg";
else if(screen.width < 1601) bg="http://www.oubien.com/furious/web/bg_1600.jpg";

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