fancybox gallery with image rollover

fancybox gallery with image rollover

I am building an image gallery with jquery fancybox to show before and after photo-retouching. 
I would like to add an image rollover effect to the large image in the fancybox overlay using the script below.  
When I apply the class="rollover" to the <a> tag the rollover script breaks. The script works fine when I apply the class"rollover" to the <img> tag. However this places a rollover effect on the gallery thumbnail.   
Can someone point me how to achieve this rollover effect on the large image please.

//fancybox gallery item
<div class="grid-element">
<a rel="example_group" href="editorial/01_editorial_lg_off.jpg" title="">      // large image
<img id="grid-shadow" src="editorial/01_editorial_sm.jpg" alt=""/></a>      //thumbnail
</div>


//rollover script
  <script type="text/javascript">
  $(document).ready(function() {
$("img.rollover").hover(          
 function() { this.src = this.src.replace("_off", "_on"); 
 }, 
 function() { this.src = this.src.replace("_on", "_off"); 
 });
  }); 
</script>

Changing the expression 
$("img.rollover").hover( 
to 
$("a.rollover").hover(   
does not work