Step1. open file on location given below

app/design/frontend/default/default/template/catalog/product/view/media.phtml

Step2. Search the code in media.phtml file

<a href="#" onclick="popWin('<?php echo $this->getGalleryUrl($_image) ?>', 'gallery', 'width=300,height=300,left=0,top=0,location=no,status=yes,scrollbars=yes,resizable=yes'); return false;" title="<?php echo $this->htmlEscape($_image->getLabel()) ?>"><img src="<?php echo $this->helper('catalog/image')->init($this->getProduct(), 'thumbnail', $_image->getFile())->resize(56); ?>" width="56" height="56" alt="<?php echo $this->htmlEscape($_image->getLabel()) ?>" /></a>

Step 3. Replace searched code with code below.

<a href="#" onmouseover="$('image').src = this.href; return false;" title="<?php echo $this->htmlEscape($_image->getLabel()) ?>"><img src="<?php echo $this->helper('catalog/image')->init($this->getProduct(), 'thumbnail', $_image->getFile())->resize(56); ?>" width="56" height="56" alt="<?php echo $this->htmlEscape($_image->getLabel()) ?>" /></a>