You can use a lightbox to do it.

There is a simple example in this link:

http://lokeshdhakar.com/projects/lightbox2/

Instead of the onclick attribute, you could assign a class popup_image to your img and attach a click handler when the DOM is ready.

I took the liberty to remove scripts and tags that were not neccessary to demonstrate the result.

$(document).ready(function() {

  $(".popup_image").on('click', function() {
    w2popup.open({
      title: 'Image',
      body: '<div class="w2ui-centered"><img src="' + $(this).attr('src') + '"></img></div>'
    });
  });

});

<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.11.3/jquery.min.js"></script>
<script type="text/javascript" src="http://w2ui.com/src/w2ui-1.4.2.min.js"></script>
<link rel="stylesheet" type="text/css" href="http://w2ui.com/src/w2ui-1.4.2.min.css" />

<img class="btn popup_image" style="width:100px; height:100px; border-radius:4px;" src="http://lorempixel.com/g/200/200/"></img>

Just add the onclick attribute

<img src="images/photostack.png" onclick="alert('Hello World')" alt="" />

Or use jQuery:

$( "#img" ).click(function() {
  alert( "Hello World jQuery" );
});

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>

<img id="img" src="images/photostack.png" alt="" />