Untitled

<ul>

  <li><img src="http://placekitten.com/50/50" /></li>

  <li><img src="http://placekitten.com/50/100" /></li>

  <li><img src="http://placekitten.com/50/50" /></li>

  <li><img src="http://placekitten.com/50/100" /></li>

  <li><img src="http://placekitten.com/50/50" /></li>

  <li><img src="http://placekitten.com/50/100" /></li>

  <li><img src="http://placekitten.com/50/50" /></li>

  <li><img src="http://placekitten.com/50/100" /></li>

</ul>

ul {

  -webkit-columns: 50px;

  -moz-columns: 50px;

  columns: 50px;

}