Magento:: Change product image on hover in catalog page

To show different image on hover on magento product category page  you have to change the small code in the list.phtml file located at 

app->design->frontend->default->your theme->template->catalog->product->list.pthml 

and in this file around line 124 you will see something like this  // in magneto 1.9 line number could be different according to magneto version

<a href="<?php echo $_product->getProductUrl() ?>" title="<?php echo $this->stripTags($this->getImageLabel($_product, 'small_image'), null, true) ?>" class="product-image">
    <?php $_imgSize = 210; ?>
    <img id="product-collection-image-<?php echo $_product->getId(); ?>"
        src="<?php echo $this->helper('catalog/image')->init($_product, 'small_image')->resize($_imgSize); ?>"
        alt="<?php echo $this->stripTags($this->getImageLabel($_product, 'small_image'), null, true) ?>" />
 </a>

just search for this and replace it with

<a href="<?php echo $_product->getProductUrl() ?>" title="<?php echo $this->stripTags($this->getImageLabel($_product, 'small_image'), null, true) ?>" class="product-image">
    <?php $_imgSize = 210; ?>
    <img id="product-collection-image-<?php echo $_product->getId(); ?>"
        src="<?php echo $this->helper('catalog/image')->init($_product, 'small_image')->constrainOnly(FALSE)->keepAspectRatio(TRUE)->resize($_imgSize); ?>"
        alt="<?php echo $this->stripTags($this->getImageLabel($_product, 'small_image'), null, true) ?>"
        onmouseover="this.src='<?php echo $this->helper('catalog/image')->init($_product, 'thumbnail')->constrainOnly(FALSE)->keepAspectRatio(TRUE)->keepFrame(FALSE)->resize($_imgSize) ?>';" onmouseout="this.src='<?php echo $this->helper('catalog/image')->init($_product, 'small_image')->constrainOnly(FALSE)->keepAspectRatio(TRUE)->keepFrame(FALSE)->resize($_imgSize) ?>';"/>
</a>

Now in admin assign different image for thumbnail and small image. thumbnail image will show when you mouse over on product image.