Oh wow. It also sounds like the girl accidentally tickled her toes and then when she realised her friend was ticklish, proceeded to deliberately tickle her haha.
The funny thing is, whilst she isn't being tickled in that photo, she does look as if she is in the middle of flinching from someone poking her sides. :D
That is another thing for me, when someone talks about the fact they were tickled. My wife has done this a few times and it always triggers me so to speak.