function randomImg() {
  var img = new Array();
img[0] = "http://moonphase.sub.jp/miyabi/top-c78a.jpg";
img[1] = "http://moonphase.sub.jp/miyabi/top-c78c.jpg";

var sum = Math.floor(Math.random() * img.length);
document.write('<img src="'+ img[sum] + '" border=0 alt="紹介ページへ">');
}