var timeout = 7000;
var imgList = new Array();
var image = new Image();

function changeImage() {
    var topImage = document.getElementById("top_img");
    topImage.src = image.src;
    preloadNextImage();
    tick();
}

function preloadNextImage() {
    image.src = "UserFiles/Image/Top/" + imgList[Math.floor(Math.random() * imgList.length)];
}

function tick() {
    setTimeout("changeImage()", timeout);
}