Sorry to burst your bubble, but that's not L4D fanart, it's a painting by an artist named Jason Chan. He painted it before L4D even came out, for a conceptart.org contest at Last Man Standing.
wow, that's nuts. I wouldn't be surprised if the developers were fans of the art then:
bill was a cool teen at the time, he was too big for playgrounds
and you didn't burst my bubble, now I know the artist responsible for this awesomeness