var bg="https://i.servimg.com/u/f79/14/89/82/31/fond_p21.jpg";

if(screen.width < 801) bg="https://i.servimg.com/u/f79/14/89/82/31/fond_p22.jpg";
else if(screen.width < 1025) bg="https://i.servimg.com/u/f79/14/89/82/31/fond_p23.jpg";
else if(screen.width < 1281) bg="https://i.servimg.com/u/f79/14/89/82/31/fond_p24.jpg";
else if(screen.width < 1601) bg="https://i.servimg.com/u/f79/14/89/82/31/fond_p25.jpg";

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