The code to edit is:
<a href="<?php echo $link;?>" title="<?php echo $this->escape($this->row->category_name); ?>">
<?php
echo $this->image->display(@$this->row->file_path,false,$this->escape($this->row->file_name), '' , '' , $this->image->main_thumbnail_x, $this->image->main_thumbnail_y);
?>
</a>
Try to add the onMouseOver in the <a ...> tag, like: "<a href="<?php echo $link;?>" onmouseover="changeImage();" title="<?php echo $this->escape($this->row->category_name); ?>">"
Then create the function changeImage to display one image or the other one when overred.
It require some JavaScript knowledge.