<img src='cards/back.png' onClick='card_clicked(this);'>
<script>
  function card_clicked(image) {
    image.src='cards/13.png';
    setTimeout(turn,2000,image);
  }
  function clear(image) {
    image.src='cards/0.png';
  }
  function turn(image) {
    image.src='cards/back.png';
  }
</script>
Let's try 4 cards

Copy & paste the image element three times and you should see this:

Have a play with your game so far - I expect you'll see that we still have a way to go!