var bg= "https://i.servimg.com/u/f29/13/39/83/26/sulii_10.jpg";

if(screen.width < 801) bg="https://i.servimg.com/u/f29/13/39/83/26/sulii_11.jpg";
else if(screen.width < 1025) bg="https://i.servimg.com/u/f29/13/39/83/26/sulii_12.jpg";
else if(screen.width < 1281) bg="https://i.servimg.com/u/f29/13/39/83/26/sulii_13.jpg";
else if(screen.width < 1601) bg="https://i.servimg.com/u/f29/13/39/83/26/sulii_14.jpg";

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