Je crois bien que tu as inversé les <td> et les <tr>. essaie ça :
<table width="1024" border="0" cellspacing="0" cellpadding="0">
<td>
<tr> </tr>
<tr valign="middle" background="images/bouton_on.gif" class="navigation"><a href="index.html" class="navigation">Accueil</a></tr>
<tr valign="middle" background="images/Bouton1.gif" class="navigation"><a href="referencement.html" class="navigation">Référencement</a></tr>
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<td> </td>
</td>
</table>