Frankly, I'm not even sure she's being tickled at all. The guy's hands aren't in the right position for that. It's possible that they're goofing off and the girl just finds it absolutely hilarious for some reason. Then again, I may be wrong.
the guys hands are not really in the perfect place, but there´s a second person on the bed. Its possible the guy in the picture was just holding her in place for the second person 🙂
haha i never thought of that idea! its definitely possible. whatever technique is being used, its obviously effective. i once had an ex-gf who would go spastic when you squeezed her knees and she looked just like the girl in this pic.