media.phtml

    <?php
    $_product = $this->getProduct();
    $_helper = $this->helper('catalog/output');
    ?>
    ...
    <?php foreach ($this->getGalleryImages() as $_image): ?>
        <li>
            <a href="#" title="<?php echo $this->escapeHtml($_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 $this->escapeHtml($_image->getLabel()) ?>" />
            </a>
        </li>
    <?php endforeach; ?>