You can emulate fixed background position unfortunately not supported by IE6 (see nerdposeur's answer) with careful "manual" positioning using background-position. Position the big image with 0,0 offset. Use the same image for selected tab, but offset it to the left and up by exactly the position of the top left corner of the tab. That will ensure exact matching of the two backgrounds you want.
You seem to have a fixed menu, so it means carefully writing background CSS for your four menu elements, one by one. Of course, if your menu is dynamic, this approach does not work. Here's a demo I quickly cooked up starting with this page:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
<head>
<title>CSS Tabbed Navigation</title>
<meta http-equiv="content-type" content="text/html; charset=utf-8" />
<style type="text/css">
body {
margin: 20px;
padding: 0px;
background: #CACCB4;
font: 16px arial, sans-serif;
background-image: url('http://www.graphicsarcade.com/backgrounds/strips/background_3.gif');
}
pre {text-indent: 30px}
#tabmenu {
color: #000;
border-bottom: 2px solid black;
margin: 12px 0px 0px 0px;
padding: 0px;
z-index: 1;
padding-left: 10px }
#tabmenu li {
display: inline;
overflow: hidden;
list-style-type: none; }
#tabmenu a, a.active {
color: #DEDECF;
background: #898B5E;
font: bold 1em "Trebuchet MS", Arial, sans-serif;
border: 2px solid black;
padding: 2px 5px 0px 5px;
margin: 0px;
text-decoration: none; }
#tabmenu a.active {
background-image: url('http://www.graphicsarcade.com/backgrounds/strips/background_3.gif');
background-position: -125px -18px;
border-bottom: 3px solid #ABAD85; }
#content {font: 0.9em/1.3em "bitstream vera sans", verdana, sans-serif;
text-align: justify;
background: #ABAD85;
padding: 20px;
border: 2px solid black;
border-top: none;
z-index: 2; }
#content a {
text-decoration: none;
color: #E8E9BE; }
#content a:hover { background: #898B5E; }
</style>
</head>
<body>
<ul id="tabmenu">
<li><a href="tab1.html">Enormous</a></li>
<li><a class="active" href="tab2.html">Flared</a></li>
<li><a href="tab3.html">Nostrils</a></li>
</ul>
<div id="content">
<p>If one examines subpatriarchialist material theory, one is faced with a choice: either accept the presemioticist paradigm of reality or conclude that the task of the artist is deconstruction, given that reality is equal to art. It could be said that the subject is contextualised into a Batailleist 'powerful communication' that includes language as a totality. Marx uses the term 'precapitalist semiotic theory' to denote the bridge between narrativity and society.</p>
<p>Any number of desituationisms concerning Sartreist absurdity may be discovered. In a sense, the textual paradigm of consensus states that reality has significance. Baudrillard uses the term 'surrealism' to denote the absurdity, and subsequent rubicon, of substructuralist class. It could be said that la Tournier[4] holds that the works of Pynchon are modernistic. The premise of the textual paradigm of consensus states that the significance of the observer is social comment. However, in Gravity's Rainbow, Pynchon examines textual materialism; in The Crying of Lot 49 he denies subcultural discourse.</p>
<br />
</div>
</body>
</html>