demo.html

<ul class="clean-list">
  <li class="thumbnail">
  <img src="http://placehold.it/100x100">
  </li>
  <li class="thumbnail">
  <img src="http://placehold.it/200x100">
  </li>
  <li class="thumbnail">
  <img src="http://placehold.it/100x200">
  </li>
  <li class="thumbnail">
  <img src="http://placehold.it/100x100">
  </li>
  <li class="thumbnail">
  <img src="http://placehold.it/100x100">
  </li>
  <li class="thumbnail">
  <img src="http://placehold.it/100x100">
  </li>
  <li class="thumbnail">
  <img src="http://placehold.it/100x100">
  </li>
</ul>

<style>
.clean-list {
  list-style: none;
  list-style-type: none;
  margin: 0;
  padding: 0;
  outline: 1px solid blue;
}

.thumbnail {
  display: inline-block;
  margin: 45px;
  width: 100px;
  height: 100px;
  text-align: center;
}

.thumbnail img:before {
  height: 100%;
  content: '\0020';
  display: inline-block;
  vertical-align: middle;
}

.thumbnail img {
  max-width: 100%;
  max-height: 100%;
  vertical-align: middle;
  display: inline-block;
}
</style>