Mad Jack is my real name...
(What, you never heard of a guy named Madalyne before?)
Okay, Sir John Garrett, who became known as "Mad Jack", was an early D&D character...
My first name is the Irish version of John, and my Catholic Confirmation name is John, so I'd started going by the name Jack in certain social circles as a teenager, and quite a few people actually thought that was my real name.
(On two additional entirely separate but extremely coincidental notes, I'd picked up the nickname "Ripper" from some acquaintances during my early teenage years, due to both my skill at driving (despite being years away from being old enough) and my constantly having one or more blades on me. And later on, a particularly literate friend compared me to Lord Byron (who'd been described as "Mad, bad, and dangerous to know") and it sort of stuck.)
(Fun Fact: A friend's young daughter once pointed out that if you say "Mad Jack" seven times really fast it turns into "Magic", lol...)
By the time I grown out of my teenage years, "Mad Jack" had sort of become my alter ego, almost a Jekyll and Hyde sort of thing. I've been using Mad Jack or some variation on it as an online persona pretty much everywhere online since I first got on the internet.
The skull is a cropped version of a photograph I took. It's an almost life-sized resin skull covered in Celtic knotwork that my brother got me for a birthday present one year, wearing my favorite hat.
Just as with Mad Jack, the skull is the picture I use for my avatar everywhere online.