That, my friend, is an upside-down star inscribed within a circle, otherwise known as a pentagram. Pentagrams in today's culture are often related to or associated with demonic or Satanic rituals. As for why @ItsJerry decided to watermark one over his face and demon-ify his profile picture, it was a joke, a pun, made at the fact he had 666 messages in total on the forums.