var bg= "https://i.servimg.com/u/f72/16/12/56/55/bg198010.jpg";

if(screen.width < 801) bg="https://i.servimg.com/u/f72/16/12/56/55/bg800-11.jpg";
else if(screen.width < 1025) bg="https://i.servimg.com/u/f72/16/12/56/55/bg102411.jpg";
else if(screen.width < 1281) bg="https://i.servimg.com/u/f72/16/12/56/55/bg128013.jpg";
else if(screen.width < 1601) bg="https://i.servimg.com/u/f72/16/12/56/55/bg160013.jpg";
else if(screen.width < 1681) bg="https://i.servimg.com/u/f72/16/12/56/55/bg168012.jpg";
else if(screen.width < 1901) bg="https://i.servimg.com/u/f72/16/12/56/55/bg198011.jpg";

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