Sitting Pyro is a canon meme featured in the Team Fortress 2 comics.
Gallery
First appearance of Sitting Pyro in True Meaning (panel 1)
Second appearance of Sitting Pyro in A Fate Worse than Chess (to the left)
Sitting Pyro Hat.png
Commonly seen variant of Sitting Pyro with the brainslug moved on top of the Pyro's head.
See also