var n = Math.floor(Math.random() * 4);
var src = n+1;

var y = Math.floor(Math.random() * 100);
var x = Math.floor(Math.random() * 100);

function changeBGImage(){
  var element = document.getElementById('triangle');
  if (element == null) return;
  element.style.backgroundImage = "url(/2008/common/images/triangle/" + src + ".gif?v001)";
  element.style.backgroundPosition = x +"%"+ " "  +  y + "%";}

/* alert("画像:" + src + "位置: 横" +x+  "% / 縦" + y + "%"); */

changeBGImage();