title: display an image only when button is clicked
body: <p>Just beginning to learn HTML &amp; Javascript.
  I have the following code, which works. however, because I have have an img tag in my body it is trying to show a place holder for an image before I click the button. How can I stop this.</p>
  
  <pre><code>&lt;!DOCTYPE HTML&gt;
  &lt;html&gt;
  &lt;head&gt;
      &lt;meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /&gt;
      &lt;title&gt;Tesco JSONP&lt;/title&gt;
  
      &lt;script type="text/javascript"&gt;
  
          function picture(){ 
          var pic = "http://img.tesco.com/Groceries/pi/118/5000175411118/IDShot_90x90.jpg"
          document.getElementById('bigpic').src = pic.replace('90x90', '225x225');
  
          }
  
  
      &lt;/script&gt;
  
  
  &lt;/head&gt;
  
  &lt;body&gt;
  
  
          &lt;img id="bigpic" src="bigpic" /&gt;
  
      &lt;button onclick="picture()"&gt;Enlarge&lt;/button&gt;
  
  &lt;/body&gt;
  
  &lt;/html&gt;
  </code></pre>
  
  <p>Best wishes.</p>