var blnDOMSUPPORT = (document.getElementById) ? true : false;
window.onload = window_load;

function window_load()
   {

   if ( blnDOMSUPPORT )
      {

      document.getElementById('icon1').onclick = swapimage;
      document.getElementById('icon2').onclick = swapimage;
      document.getElementById('icon3').onclick = swapimage;
      
      }
   else
      {

      document.images['icon1'].onclick = swapimage;
      document.images['icon2'].onclick = swapimage;
      document.images['icon3'].onclick = swapimage;

      }

   }

function swapimage()
   {
   
   if ( blnDOMSUPPORT )
      {

      var imgMain = document.getElementById('mainimage');
      var divParent;

      // create new image element
      var imgNew = document.createElement('img');

      // give it an image
      // here I use the src of the icon just for convenience
      // you'd want to somehow determine the src of your desired image
      imgNew.src = this.src;
   
      // give it an id
      imgNew.id = 'mainimage';

      // replace image
      divParent = imgMain.parentNode;
      divParent.replaceChild(imgNew, imgMain);

      }
   else
      {

      // old school image swap
      document.images['mainimage'].src = this.src;

      alert( 'old school' );

      }
   
   }         

<div id="main">
<img id="mainimage" src="http://www.w3schools.com/css/smiley.gif">
</div>
<div id="icons">
<img id="icon1" src="http://www.w3schools.com/css/smiley.gif">
<img id="icon2" src="http://www.w3schools.com/images/vxhtml.gif">
<img id="icon3" src="http://www.w3schools.com/images/print.gif">
<input type="button" value="Toggle DOM Support" onclick="window.blnDOMSUPPORT = !window.blnDOMSUPPORT;">
</div>