Display:Block problem

HTML

<div class="image_holder">
  <a href=""><img src="http://imgur.com/a/hf7jx" width="180" height="200"></a>
</div>

<div class="image_holder2">
  <a href=""><img src="http://imgur.com/a/hf7jx" width="180" height="200"></a>
</div>

CSS

.image_holder {
  margin:20px 0px;
  background-color: aquamarine;
}

.image_holder img {
  display: block;
}

.image_holder2 {
  margin:20px 0px;
  background-color: aquamarine;
}

.image_holder2 img {
  display: inline-block;
}