I want to use rollover images for the horizontal menu bar.

This code works:
<li><?php echo '<a href="' . HTTP_SERVER . DIR_WS_CATALOG . '" onMouseover="MyLink1.src=\'includes/templates/menu-home-on.png\';" onMouseOut="MyLink1.src=\'includes/templates/menu-home-off.png\';">'; ?><img id="MyLink1" src="includes/templates/menu-home-off.png"></a></li>
...but I would prefer to use a variable so that the images could be stored in the working template folder and that this code could be used for other templates without changing the code.

So - is there a simple variable I can drop in there that points to the MY_TEMPLATE directory?