<?php if($category->getThumbnail()): ?>
    <a href="<?php echo $category->getURL(); ?>" title="<?php echo $category->getName(); ?>">
    <img src="<?php echo Mage::getBaseUrl('media').'catalog/category/'. $category->getThumbnail(); ?>" alt=""  />
    </a>
<?php endif; ?> 

在分类页.phtml 调取