form表单上传图片上传不了

 <body>

  <div class="container">
     <div class="clear"></div>
    
     <div id="forms" class="mt10">
        <div class="box">
          <div class="box_border">
            <div class="box_top"><b class="pl15">添加会员：</b></div>
            <div class="box_center">
             <table class="form_table pt15 pb15" width="100%" border="0" cellpadding="0" cellspacing="0">
  <form enctype="multipart/form-data"action="./action.php?a=add" method="post" class="jqtransform">
              <tr>
                  <td class="td_right">用户名：</td>
                  <td class=""> 
                    <input type="text" name="username" class="input-text lh30" size="20">
                  </td>
              </tr>
   <tr>
                  <td class="td_right">密码：</td>
                  <td class=""> 
                    <input type="password" name="userpwd" class="input-text lh30" size="20">
                  </td>
             </tr>
     <tr>
                  <td class="td_right">确认密码：</td>
                  <td class=""> 
                    <input type="password" name="userrepwd" class="input-text lh30" size="20">
                  </td>
               </tr>
         <tr>
                   <td class="td_right">用户头像：</td>
                   <td><input type="file" name="userpic"></td>
                </tr>
         <tr>
                  <td class="td_right">用户昵称：</td>
                  <td class=""> 
                    <input type="text" name="nickname" class="input-text lh30" size="20">
                  </td>
                </tr>
                <tr >
                  <td class="td_right">用户等级：</td>
                  <td class="">
                    <span class="fl">
                      <div class="select_border"> 
                        <div class="select_containers "> 
                        <select name="status" class="select"> 
<option value="0">普通用户</option> 
<option value="1">管理员</option> 
<option value="2">青铜</option> 
<option value="3">白银</option> 
<option value="4">黄金</option> 
                        </select> 
                        </div> 
                      </div> 
                    </span>
                  </td>
                 </tr>
                 <tr>
                   <td class="td_right">&nbsp;</td>
                   <td class="">
                     <input type="submit" name="submit" class="btn btn82 btn_save2" value="添加"> 
                    <input type="reset" name="reset" class="btn btn82 btn_res" value="重置"> 
                   </td>
                 </tr>
                </form>
               </table>
            </div>
          </div>
        </div>
     </div>
   </div> 
 </body>
提交到action.php页面，打印$_POST['userpic']始终显示为空