These "images" are icons fonts. They are usually added via :before/:after pseudo elements. In this instance, the content value is an ASCII representation of an external font library character.
.icon-flag:before { content: "\f024"; }
In order for this to work, you would need to change the element's font-family property to reference the external font library. In your case, the font library is FontAwesome.
[class^="icon-"]:before, [class*=" icon-"]:before { font-family: FontAwesome; font-weight: normal; font-style: normal; display: inline-block; text-decoration: inherit; }
Using the Font-Awesome library, you could simply add an icon like this:
<i class="fa fa-stack-overflow"></i>
Since it's treated like font, you can increase the size of it using the CSS property font-size. (example)
.fa-stack-overflow { font-size:30px; color:orange; }