Listbox ile İlgili Bir Sorum Var

<script type="text/javascript">

function imgload(id){
if(id=='istanbul'){
	document.getElementById('listimg').src = 'http://www.facebookthe.com/fotograf/2011/04/istanbul-2.jpg';
	}else if(id=='ankara'){
	document.getElementById('listimg').src = 'http://yesilcimen.com/wp-content/uploads/ankara3.jpg';	
		}else if(id=='izmir'){
		document.getElementById('listimg').src = 'http://img03.blogcu.com/images/a/t/m/atmaz/f3ee5548983d8e8de308402886a3d9e2_1291283552.jpg';	
			}
}
</script><form name="form1" method="post" action="">
  <label for="liste"></label>
  <table width="433" border="0" cellspacing="0" cellpadding="0">
    <tr>
      <td width="215">
      <select name="liste" id="liste" onChange="imgload(this.value);">
        <option value="istanbul">İstanbul</option>
        <option value="ankara">Ankara</option>
        <option value="izmir">İzmir</option>
      </select></td>
      <td width="218"><img src="http://www.osmanoguz.net/resim/google.jpg" name="listimg" width="200" height="200" id="listimg"></td>
    </tr>
  </table>
</form>