①写个Js 方法叫做ChangeImg

<script language="javascript" type="text/javascript">
  function ChangeImg(id)
  {
   var userface = id;
   document.getElementById("imgFace").src = "../images/userface/"+ userface.options[userface.selectedIndex].value+".jpg"
  }
 </script>

②body里面的内容

<select name="userface" size="1" onchange="ChangeImg(this)" runat="server">
     <option value="01" >01</option>
     <option value="02">02</option>
     <option value="03">03</option>
     <option value="04">04</option>
     </select>&nbsp;<img ID="imgFace" runat="server" class="face" src="../images/userface/01.jpg" />