I vaguely recall looking into this back when I set this place up. It's not something I could easily figure out how to do (IIRC I just replaced the default vBulletin images with these modified ones).
That being said, it's not a "FreeBSD logo", it's the generic BSD logo which dates back to the '70s, long before FreeBSD came into existence.
The "FreeBSD logo" is the
stylized daemon, which was chosen back in ~2005 exactly because the daemon represents an entire family of systems, rather than just FreeBSD.