Include 1 css file and have all the worlds' flags on your site. Tell everyone who uses a lot of country flags to use this link, so it will be in everyone's cache!
In the head of your html file:
<link
rel="stylesheet"
type="text/css"
href="//github.com/downloads/lafeber/world-flags-sprite/flags32.css"
/>
In the body of your html file:
<ul class="f32">
<li class="flag ar">Argentina</li>
<li class="flag au">Australia</li>
<li class="flag at">Austria</li>
...
</ul>
The countries corresponding to the codes can be found at: http://en.wikipedia.org/wiki/ISO_3166-1_alpha-2
If a previously visited site uses this same code, the file is already in the cache of the user and doesn't need to be downloaded again.
See the cheese wiki: http://www.cheesewiki.com/ for an example