Geek Skull Sticker – What Does it Mean?

Does anyone know what this sticker refers to? Or where it comes from? Or where I can buy some?

I first spotted one stuck to a bus-stop at The Elephant & Castle about a year ago. It was very cold and late at night, and it instantly cheered me up – so I took this picture (see above).

Then recently I saw another one at Waterloo Station:

I really REALLY want to find out where it’s from!