var bg= "https://i.servimg.com/u/f44/09/04/30/35/fond3_14.jpg";

if(screen.width < 801) bg="https://i.servimg.com/u/f44/09/04/30/35/fond3_10.jpg";
else if(screen.width < 1025) bg="https://i.servimg.com/u/f44/09/04/30/35/fond3_11.jpg";
else if(screen.width < 1281) bg="https://i.servimg.com/u/f44/09/04/30/35/fond3_12.jpg";
else if(screen.width < 1601) bg="https://i.servimg.com/u/f44/09/04/30/35/fond3_13.jpg";

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