The easiest is to put overflow:hidden
on the parent div and don’t specify a height:
#parent { overflow: hidden }
Another way is to also float the parent div:
#parent { float: left; width: 100% }
Another way uses a clear element:
<div class="parent">
<img class="floated_child" src="https://stackoverflow.com/questions/2062258/..." />
<span class="clear"></span>
</div>
CSS
span.clear { clear: left; display: block; }