var bg= "http://i.imgur.com/soh0I.jpg";

    if(screen.width < 801) bg="http://i.imgur.com/ZcNLs.jpg";
    else if(screen.width < 1025) bg="http://i.imgur.com/ihHCD.jpg";
    else if(screen.width < 1281) bg="http://i.imgur.com/YCwIB.jpg";
    else if(screen.width < 1601) bg="http://i.imgur.com/soh0I.jpg";

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