var bg= "https://i.servimg.com/u/f46/16/66/98/42/bg310.jpg";

if(screen.width < 801) bg="https://i.servimg.com/u/f46/16/66/98/42/bg_80010.jpg";
else if(screen.width < 1025) bg="https://i.servimg.com/u/f46/16/66/98/42/bg_10210.jpg";
else if(screen.width < 1281) bg="https://i.servimg.com/u/f46/16/66/98/42/1280x910.jpg";
else if(screen.width < 1441) bg="https://i.servimg.com/u/f46/16/66/98/42/1440x110.jpg";
else if(screen.width < 1601) bg="https://i.servimg.com/u/f46/16/66/98/42/bg310.jpg";

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