The instructions on http://twittercounter.com/pages/buttons page says you need to use the script at the bottom in order to post a box on your blog.
I quickly took a look at the script generated for the nice bird icon and realized they are using php to modify the image with information passed in the image url, very clever.
<a href='http://twittercounter.com/codepyro?from=button' target='_new' title='TwitterCounter for @codepyro'>
<img alt='TwitterCounter for @codepyro' height='90' src='http://srv2.twittercounter.com/counter/index_bird.php?username=codepyro' style='border:none;' width='149'/>
Then paste the code into your blog, i used the expand widgets checkbox and posted the twitterFollower code in my profile section.