var rotation_size = 23;
var week = 7 * 24 * 60 * 60 * 1000;

function getRotationImage()
 {
  var date = new Date();
  var time = date.getTime();
  var weeks = Math.round(time/week);
  var num = (weeks % rotation_size) + 1;
  var image = "images/rotation/" + ((num < 10)? "0" : "") + num + ".jpg";

  document.write("<img class='framed' src='" + image + "' style='margin: 15px 0 0 0' />");
 }