For this (smaller) button:
use the following code:
<a href="http://planetatheism.com/" title="Planet Atheism"><img src="http://planetatheism.com/buttons/pa-small.gif" alt="Planet Atheism - aggregating blogs by non-believers and freethinkers"/></a>
For this (larger) button:
use the following code:
<a href="http://planetatheism.com/" title="Planet Atheism"><img src="http://planetatheism.com/buttons/pa-large.gif" alt="Planet Atheism - aggregating blogs by non-believers and freethinkers"/></a>