Javascript – HTML – onclick does not work on img

In my HTML web page I have an image and I would like to trigger a javascript function when the image is clicked. Here is the codes:

<img src="pond1.jpg" height="150" size="150" alt="Johnson Pond" onclick="imagefunction()">

<script>
 function imagefunction() {
      alert("Hello world") 
      }
</script>

The problem is that by clicking on the image nothing happens.

How can we get onclick event on img tag?