T'is an ancient practice amongst the first generations of Brawlers: Brawl cards. They're a stylized, automatically generated image for your signature that displays your Brawl FC, two mains (a Primary with a large portrait, and a secondary with a smaller portrait), and your in-game name. They could be "leveled up", t' distinguish newbies (who only recently got their card, and thus a shitty level) from experienced players. Used one myself back in the day, 'fore Wiichat's Brawl scene up 'n died.
... Ain't gonna lie, I miss havin' Wolf's mug in my sig.