Let me preface this by saying that most of this is conjecture. So thinking back to some memes that I’ve observed were notably popular among some leftists on Tumblr, there was the Gadsden flag meme and the gun meme. I think the Gadsden flag meme was attractive to leftists because (1) it was a means to irritate right-wingers and reactionaries and (2) the use of crude graphic design had an absurdist shitposting quality about it. I think this second point is the more universal one when it comes to memes and leftism on Tumblr as not all memes necessarily have a quality that is irritating to right-wingers and reactionaries.
With regard to the second point made, there was an essay that circulated some time ago on Tumblr about the relationship between shitposting and communism. Although the essay itself does not make much reference to actual leftist ideology, it does provide a thought-provoking thesis–that shitposting “create[s] a uniquely relatable and political literary body of work which strives to process and respond to the anxieties of a life intertwined with the vast and overwhelming mechanizations of a failing capitalist society.” (The essay can be read in full here.) If we were to accept this thesis as true, we can see why, for example, the gun meme has been popular among some leftists on Tumblr. The sheer non-sequitur / absurdist quality makes it have a close association with the shitposting genre.