イマジンもsubmitです

<script type="text/javascript">
    function aa()
    {
      var actionUrl = registerUser.action';
      document.forms[0].action = actionUrl;
      document.forms[0].submit();
       }
  </script>
<s:form action="registerUser" method="post" theme="simple">
 <table width="303" border="0" cellspacing="3" cellpadding="0">
 <tr>
      <td colspan="2" class="cont_font"><br/>
        <input type="image" name="imageField" src="include/img/button5.jpg" onclick="aa()"/>
      </td>
    </tr>
</table>