function banner() {
};

banner = new banner();
number = 0;

// bannerArray
banner[number++] = "<a href='https://secure.gcash.com/jsp/gc/0/gcUserPass.jsp' target='_blank'><img src='http://www.galaxiworld.com/images/entry_image_1.jpg' border='0'></a>"
// keep adding items here...

increment = Math.floor(Math.random() * number);

document.write(banner[increment]);