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 58 you will see something like this
<a href="<?php echo $_product->getProductUrl() ?>" title="<?php echo $this->stripTags($this->getImageLabel($_product, 'small_image'), null, true) ?>" class="product-image"><img src="<?php echo $this->helper('catalog/image')->init($_product, 'small_image')->resize(135); ?>" width="135" height="135" 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"><img src="<?php echo $this->helper('catalog/image')->init($_product, 'small_image')->constrainOnly(FALSE)->keepAspectRatio(TRUE)->keepFrame(FALSE)->resize(135); ?>" width="135" height="135" 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(135) ?>';" onmouseout="this.src='<?php echo $this->helper('catalog/image')->init($_product, 'small_image')->constrainOnly(FALSE)->keepAspectRatio(TRUE)->keepFrame(FALSE)->resize(135) ?>';" />
that’s all we need to do
note in list file there are two modes list and grid above will work for list mode to make it work in grid mode change it around line 95 in same file
