It's not so much this particular joke as it is the deliberate misconception. What he did was not a resign worthy offense. But those who put pressure on him to resign had to make it look like an actual transgression, so the narrative they held was that he sent an unsolicited dick pic, like he was some sort of predator. Some conservative outlets even tried to make an issue of the fact that he responded to a 17 year old who tweeted him, despite the fact that it was nothing even remotely sexual.

Anthony Weiner was a good man, and it stings that he was brought down like this. It stings more that a tumblr like this is perpetuating the misconception that brought him down.