如何在侧边栏上放置4个缩略图

Question

我有一个侧边栏是col-md-2。现在我需要在这个侧边栏上放置4个缩略图，它们的图片会响应。当我调整窗口大小（使窗口变小）时，此代码图像会变大;

    <div class=col-md-2>
   <div class=row>

   <div class=col-md-3>
   <div class="thumbnail>
   <a href="#">
   <img srcset="address" class="img-responsive">
   <div class="caption">
   a
   </div>
   </a>
   </div>
   </div>

  <div class=col-md-3>
   <div class="thumbnail>
   <a href="#">
   <img srcset="address" class="img-responsive">
   <div class="caption">
  a
   </div>
   </a>
   </div>
   </div>

   <div class=col-md-3>
   <div class="thumbnail>
   <a href="#">
   <img srcset="address" class="img-responsive">
   <div class="caption">
  a
   </div>
   </a>
   </div>
   </div>

   <div class=col-md-3>
   <div class="thumbnail>
   <a href="#">
   <img srcset="address" class="img-responsive">
   <div class="caption">
   a
   </div>
   </a>
   </div>
   </div>

  <div><!--end of row>
  <div>