Code java script

Bonjour,
voila mon code pour faire un rollover sur mon image mais ca ne marche toujours pas. Je ne vois plus mon image

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="fr" >
<head>
<title>index emiline</title>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" />
<link rel="stylesheet" media="screen" type="text/css" title="Exemple" href="design_emeline4.css" />
<!-- DEBUT DU SCRIPT -->
<SCRIPT LANGUAGE="JavaScript">
function rollover {
var img = document.getElementById("id_presentation");
if(img.src == "image/presentation_on") {
img.src = "images/presentation_off";
} else {
img.src = "image/presentation_on";
}
}
</SCRIPT>
<!-- FIN DU SCRIPT -->

</head>

<body>

<!-- fond -->

<div id="unite">
<div class="en_tete">
<img src="image/bande_haut_index.jpg">
</div>
<div class="menu">
<img src="image/gauche_bouton.jpg">

<img src="images/presentation_off" id="id_presentation" onmouseover="rollover()" onmouseout="rollover()" />

<img src="image/jourj_off.jpg">
<img src="image/infos_pratiques_off.jpg">
<img src="image/contact_off.jpg">
<img src="image/droite_bouton.jpg">

</div>
<div class="bas_page">
<img src="image/bande_bas_index.jpg">
</div>
</div>

</body>
</html>

sinon la page c'est

http://www.sandrineetgwen.fr/emeline4.html

Le problème vient d'ou?
merci d'avance

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