Magento Product Thumbnail Image Switcher

Step:1

go to file media.phtml
'app/design/frontend/default/default/template/catalog/product/view/media.phtml'


step:2
Find the code below
<a href="#" onclick="popWin('<?php echo $this->getGalleryUrl($_image) ?>', 'gallery', 'width=300,height=300,left=50,top=50,location=no,status=yes,scrollbars=yes,resizable=yes'); return false;">
  </a>
Replace it with
<a href="<?php echo $this->helper('catalog/image')->init($this->getProduct(), 'image', $_image->getFile()); ?>" title="<?php echo $_product->getName();?>" onclick="$('image').src = this.href; return false;">
   <!--nested img tag stays the same-->
</a>