yep, the code that you are looking for is here:
modules/Manufacturers/menu_manufacturers.tpl
after you are in there, to add the triangle icon in front of the manufacturers all you would need to do is change this code
{if $show_other_manufacturers}
<li><a href="manufacturers.php">{$lng.lbl_other_manufactu rers}</a></li>
{/if}
to
{if $show_other_manufacturers}
<li>
<img src="images/triangle.gif"><a href="manufacturers.php">{$lng.lbl_other_manufactu rers}</a></li>
{/if}
just make sure you change my line of code in red :
<img src="images/triangle.gif"> to wherever your triangle image is located at.