Problem with "sliding doors" #active

Hi!
I´m struggling with a top menu, "sliding doors" with CSS.
I have two pictures with rounded corners. One left and one right.
I´m trying to highlight my links when selected with #active_menu-nav
but only one picture is visible. (Highlight is enabled in topmenu module)

I attached a screenshot with the problem and as you can see the selected link only have one rounded corner, i want the left-pic to be shown aswell.

here is the CSS code.

#buttons {
float: left;
margin:0px;
padding:0px;
width:100%;

}

#buttons ul {
list-style: none;
padding: 0;
margin: 0;
}

#buttons a {
background:url(../images/menu-left.png) no-repeat left top;
line-height: 20px;
height:24px;
display: block;
padding-left:12px;
padding-right: 12px;
color: #FFFFFF;
font-weight: bold;

}

#buttons li {
background:url(../images/menu-right.png) no-repeat right top;
height:24px;
padding-left: 0px;
padding-right: 0px;
float: left;
margin-left: 2px;
font-size: 12px;
white-space: nowrap;
font-weight: bold;
}

#buttons a#active_menu-nav {
background:url(../images/menu-right2.png) no-repeat right;

}
#buttons li #active_menu-nav {
background:url(../images/menu-left2.png) no-repeat left;

}

HTML code generated by joomla

<div id="buttons">
<ul id="mainlevel-nav">
<li><a href="http://localhost/joomla/index.php" class="mainlevel-nav" >Home</a></li>
<li><a href="http://localhost/joomla/index.php?option=com_virtuemart&Itemid=28" class="mainlevel-nav" id="active_menu-nav">Shop</a></li>
<li><a href="http://localhost/joomla/index.php?option=com_virtuemart&Itemid=34" class="mainlevel-nav" >Konfigurator</a></li>
<li><a href="http://localhost/joomla/index.php?option=com_content&task=section&id=1&Itemid=2" class="mainlevel-nav" >News</a></li>
<li><a href="http://localhost/joomla/index.php?option=com_contact&Itemid=3" class="mainlevel-nav" >Contact Us</a></li>
</ul>
</div>

I think this id="active_menu-nav" should be inside the <li> , but its inside the "a" tag??

It would be so nice if someone know how to fix this..

best regards
/Robert