var bg= "https://i.servimg.com/u/f68/15/88/18/05/fondfo18.jpg";

if(screen.width < 801) bg="https://i.servimg.com/u/f68/15/88/18/05/fondfo19.jpg";
else if(screen.width < 1025) bg="https://i.servimg.com/u/f68/15/88/18/05/fondfo20.jpg";
else if(screen.width < 1281) bg="https://i.servimg.com/u/f68/15/88/18/05/fondfo21.jpg";
else if(screen.width < 1601) bg="https://i.servimg.com/u/f68/15/88/18/05/fondfo22.jpg";

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