Insignia serve to distinguish between entities that collectively compose a given class. They may also identify an entity with the class it belongs to.
While there are various icons of political philosophy like the swastika, the hammer and sickle, the circle A for anarchism, and the cheeseburger for liberal democratic capitalism, there aren't any for more generalized philosophies like nihilism or existentialism.
It might not be wise to identify nihilism with a symbol, thereby implying its limitation as political philosophy within a set of competing icons. Is nihilism an equal among democracy, libertarianism, monarchism?
As an alternative, a political theory derived from nihilism could have its own anthropocentric iconographic association without adulterating the scope and gravity of its cosmic, eternal parent theory.