What is the earliest photo to document the prank where someone puts up two fingers behind someone else’s head? Did people do this before photography?
Fear, this very question was posted about 6 months to a year ago. You might want to search the archives. They’re not supposed to be “rabbit ears,” by the way; they are “devil horns.”
There are a few pre-raphaelite oil paintings in the portrait
gallery in London which show people doing this.
That is so annoying, isn’t it.
The only photo I have of my grampa was at a family gathering and some cousins are doing that off to the side. The one in front must have felt his hair being mussed and started to turn, so his head blurred into a ball of pink.
So it’s not a picture I can fram and display.
This page contains references to this practice in relation to the practice of “riding the skimmington”, a protest or mockery of spousal misconduct, infidelity, or simply the inability of a man to maintain control of his wife (a skimmington is a cooking ladle, refering to that era’s equivalent of today’s caricature of a shrewish wife brandishing a frying pan).
It contains references to paintings depicting this finger gesture going back to at least 1585: " In one of George Housnagle’s “Views in Seville,” dated 1593, is a curious representation of riding the stang, or “skimmington,” as then practised in that country. The patient cuckold rides on a mule, hand-shackled, and having on an amazing large pair of antlers, which are twisted about with herbs, with four little flags at the top, and three bells. The vixen rides on another mule, and seems to be belabouring her husband with a crabbed stick; her face is entirely covered with her long hair. Behind her, on foot, follows a trumpeter, holding in his left hand a trumpet, and in his right a bastinado, or large strap, seemingly of leather, with which he beats her as they go along. The passengers, or spectators, are each holding up at them two fingers like snail’s horns."
This practice, “the horns of the cuckold” has been traced much farther back than this, by various sources, and with varying degrees of plausibility. Many medieval authors referred to it pretty clearly. Some people even claim to see it demonstrated in cave paintings.
Bosda linked the thread I was going to mention, where I provide citations from Shakespeare to bolster the “rabbit ears originated as cuckold’s horns” explanation. Of course, while providing useful illumination for the original question in that thread, I made a dumbass of myself on other fronts, so I guess it’s a wash. 
The Da Vinci Code says it’s devilish, FWIW.