Since the Hammer and Sickle was designed nearly a generation after his death, what would he have seen as representing Communism in posters, pictures, banners, and even graffiti?
RED FLAGS ▶ Red flags have been a symbol of revolutionary politics since the French revolution and as such weren't historically exclusive to socialism, but they became increasingly associated with socialism specifically following the 1871 uprising of the Paris commune in which red flags featured prominently. They've been a staple of left-wing protests, strikes and insurrections since then.
L'INTERNATIONALE ▶ The 1871 poem L'Internationale written by former communard Eugène Pottier and put to song by Pierre De Geyter in 1888 was fairly popular and eventually adopted as an anthem by the Socialist International, the de facto successor to the International Workingmen's Association in which Marx took part as a prominent figure.
"WORKERS OF THE WORLD, UNITE!" ▶ The famous slogan ("Proletarier aller Länder vereinigt Euch!" in the original German) exhorting the international working class to shed national divisions in pursuit of their class interests was the climatic conclusion of the 1848 Communist Manifesto, and some early editions also have it written on the cover. It was engraved on the 1957 redesign of Marx's tombstone in the Highgate Cemetery of London.