var bg= "http://mi9.com/uploads/sports/4746/football-widescreen-wallpaper_2560x1600_85670.jpg";

if(screen.width < 641) bg="";
else if(screen.width < 853) bg="http://mi9.com/uploads/sports/4746/football-widescreen-wallpaper_852x480_85670.jpg";
else if(screen.width < 1025) bg="http://mi9.com/uploads/sports/4746/football-widescreen-wallpaper_1024x768_85670.jpg";
else if(screen.width < 1153) bg="http://mi9.com/uploads/sports/4746/football-widescreen-wallpaper_1152x864_85670.jpg";
else if(screen.width < 1281) bg="http://mi9.com/uploads/sports/4746/football-widescreen-wallpaper_1280x800_85670.jpg";
else if(screen.width < 1367) bg="http://mi9.com/uploads/sports/4746/football-widescreen-wallpaper_1366x768_85670.jpg";
else if(screen.width < 1441) bg="http://mi9.com/uploads/sports/4746/football-widescreen-wallpaper_1440x900_85670.jpg";
else if(screen.width < 1601) bg="http://mi9.com/uploads/sports/4746/football-widescreen-wallpaper_1600x1200_85670.jpg";
else if(screen.width < 1681) bg="http://mi9.com/uploads/sports/4746/football-widescreen-wallpaper_1680x1050_85670.jpg";
else if(screen.width < 1921) bg="http://mi9.com/uploads/sports/4746/football-widescreen-wallpaper_1920x1200_85670.jpg";

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