function randomImage() {
    var fileNames = [
    "image1.png",
    "image2.jpg",
    "image3.png"
    ];
    var randomIndex = Math.floor(Math.random() * fileNames.length);
    document.getElementById('#background').innerHTML = 'url(' +
    fileNames[randomIndex] + ')';
    }
    randomImage();
    setInterval(randomImage, 1000);