scramble.js
function scramble() {
  var arr=[];
  for (var i=0;i<4;i++) arr[i]=i;
  Shuffle(arr);
  var i=0;
  for (var r=1;r<3;r++) {
    for (var c=1;c<3;c++) {
      var id=arr[i]; i++;
      var el=document.getElementById(id);
      el.style.gridRowStart=r;
      el.style.gridColumnStart=c;
    }
  }
}
index.html
<html>
<head>
  <title>Jigsaw</title>
  <link rel='stylesheet' href='styles.css'>
  <script src='library.js'></script>
  <script src='pieces.js'></script>
  <script src='scramble.js'></script>
</head>
<body>
  <div>
    <img src='images/squares.png'>
  </div>
  <br>
  <div id='grid'>
    <script>pieces(); scramble();</script>
  </div>
</body>
</html>