If I’m not mistaken, the banana thing started out by a Cologne artist who used to use a banana form to “paint” different motives. I remember his photo of the Cologne Dome painted only with this banana form. It was pretty funky. To show that galleris supported local artists (or whatever hidden agenda), tehy would put this banana sticker outside their gallery. It might have turned into a cult thing for all I know. You’ll see it outside the Statsgallerie in Stuttgart. I even have a banana on my desk at home!