var bg= "https://i.servimg.com/u/f30/12/20/96/66/54533911.jpg";

if(screen.width < 801) bg="https://i.servimg.com/u/f30/12/20/96/66/54533910.jp";
else if(screen.width < 1025) bg="https://i.servimg.com/u/f30/12/20/96/66/bannia10.jpg";
else if(screen.width < 1281) bg="https://i.servimg.com/u/f30/12/20/96/66/bannia11.jpg";
else if(screen.width < 1601) bg="https://i.servimg.com/u/f30/12/20/96/66/54533911.jpg";

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