The current Bitcoin logo was adopted in the reference client's version 0.5.0 in 2011 (but it wasn't until 2014 that it rebranded to the current name Bitcoin Core). The logo was designed by bitcointalk user "bitboy" here: bitcointalk.org/index.php?topic=1631.msg19455#msg19455, and was not intended to be specific for one piece of sofrware.