During the Republican debate, candidates devolved into a literal penis measuring contest. Here's a video of a choice clip of Donald Trump defending his size because as Americans, that's the kind of thing that's important to us when choosing a leader.
This lead to the creation of the "Girthers" movement, a pun on the "Birthers" movement that plagued President Obama's campaign. So, at least something funny came out of it. Of course Twitter is awash with jokes (be prepared for a lot of "long form" puns).
If you're looking for context, this banana was apparently making a statement about his mayor's Christmas use of the official mayoral vehicle to travel to Coffs Harbour, which features "the big banana" as its icon.