Please help me with this Email Form

[mishu007]

I have a Form from where user Will put the Name, Message and his mail Id. And The details will be mailed to specified Mail BOX. Now I Need to put a image instead of a Submit Button. If i put the Submit Button the Mail Easily Delivered to me. If i put the Image then its not delivering.. please help me what to do??

<?php
 $error = "";
 $success = FALSE;
if(isset($_POST['submit']))
{
  foreach($_POST as $key => $val)
  {
    if(empty($_POST[$key]))
    {
      $error .= "You must enter a value for " .
                ucwords(str_replace("_"," ",$key)) . ". ";
    }
    else
    {
      if(get_magic_quotes_gpc())
      {
        $_POST[$key] = stripslashes($val);
      }
    }
    if($key != 'enquiry')
    {
      $_POST[$key] = preg_replace('/[\r\n]/', ' ', $val);
    }
  }
   if($_POST['email'] == 0)
  {
    //$error .= "'{$_POST['email']}' does not appear to be a valid email address. ";
  }

    $to = "xxx@***.com"; 
	$headers  = "MIME-Version: 1.0\r\n"; 
	$headers .= "Content-type: text/html; charset=iso-8859-1\r\n"; 
	$headers .= "From: ".$_POST['email']."\r\n"; 
	$mail =  @mail($to,$_POST['name'],$_POST['enquiry'],$headers); 
	if(!$mail)
    {
      $error .= "There was an unknown error occurred ";
    }
    else
    {
      $error .= "Your message has been successfully sent. ";
    }
}

?>

<form name="ContactUs" method="post" action="../index.php?main_page=contact_us&action=success">
<table  width="100%" border="0" cellspacing="2" cellpadding="2" >
  <tr>
    <td class="plainBoxHeading" colspan="2">NAME</td>
  </tr>
  <tr>
    <td class="main" colspan="2"><input name="name" type="text"></td>
  </tr>
  <tr>
    <td class="plainBoxHeading" colspan="2">EMAIL</td>
  </tr>
  <tr>
    <td class="main" colspan="2"><input name="email" type="text" size="50"></td>
  </tr>
  <tr>
    <td class="plainBoxHeading" colspan="2">Message</td>
  </tr>
  <tr>
    <td class="main" colspan="2"><textarea name="enquiry" cols="50" rows="15"></textarea></td>
  </tr>
  <tr>
    <td align="right" class="main"><a href =" javascript : submitForm(); "><img src="../includes/templates/template_default/buttons/english/button_send.gif" alt="send now"></a></td>
  </tr>
</table>
</form>


<script language = "javascript">
function submitForm() {
document.ContactUs.submit();
}
</script>