Code:
<td id="navbar_search" class="vbmenu_control"><a href="search.php$session[sessionurl_q]" accesskey="4" rel="nofollow">$vbphrase[search]</a> <if condition="$show['quicksearch']"><script type="text/javascript"> vbmenu_register("navbar_search"); </script></if></td>
I lifted that code right from the default vbulletin style and then put and image in place of '$vbphrase[search].