The «Public Domain Mark» icon from <a href="https://en.wikipedia.org/wiki/en:Creative_Commons" class="extiw" title="w:en:Creative Commons">Creative Commons</a> with transparent background.
Creative Commons (modifications: smaller, 64px)
2012-04-16
<a rel="nofollow" class="external text" href="http://staging.creativecommons.org/about/downloads">Official downloads of the Creative Commons icons</a>.