It can come in many different forms. Girls who could not only kick your ass, but could destroy you. Girls who just have that aura around them even if they may not look physically overpowering.
It can be a personal aesthetic, it can be their personality, they could just "feel" badass, or maybe the artist of one specific image really did a girl justice in making them look extremely cool. There are many different ways to classify a girl as badass, so lets see them.