Drupal 7 FAPI - ajax 图像预览

I am trying to preview the uploaded image using ajax in my D7 module but don't know why it's not working. Here is what I am trying to do:-

function create_ad_form($form, &$form_state)
{
   $form['image_file'] = array(
       '#title' => t('Upload Banner:'),
       '#type' => 'file',
   );

   $form['submit'] = array(
       '#type' => 'submit',
       '#value' => 'Submit',
       '#submit' => array('create_ad_form_submit'),
       '#ajax'=> array(
          'callback'=>'create_ad_form_submit',
          'wrapper'=> 'im-area',
          'method'=> 'replace',
          'effect'=> 'fade',
       )
   );

   $form['im-container']=array(
       '#prefix'=>'<div id="im-area">',
       '#suffix'=>'</div>',
   );
}

function create_ad_form_submit($form, $form_state)
{
   $file = file_save_upload('image_file', array(), "public://",$replace = FILE_EXISTS_REPLACE);
   if ($file)
   {
       $file->status=FILE_STATUS_PERMANENT;
       file_save($file);
       $form['im-container']=array(
             '#title'=>t('Preview:'),
             '#prefix'=>'<div id="im-area">',
             '#markup'=>'<img src="sites/default/files/'.$file->filename.'">',
             '#suffix'=>'</div>',
       );
   }
   else
       drupal_set_message('No file uploaded.');

   return $form['im-container'];
}

上面的代码没有给出错误，但是图像预览没有显示出来.谁能告诉我在这里做错了什么，我该怎么办???谢谢.