The human body can be forced to do many unnatural things that can easily result in pain and injury. The really strange thing is the way people experience a source of pride in being able to do what reallly amounts to abusing their own body. Obviously, they don't realize the risk they are taking until it's too late.
Personally, I would be very reluctant to put a person's body in any position of extreme stress such as the one in the picture and then tickle them. If I did do it I would limit the tickling to just a few seconds. I would also try to provide a maximum amount of external support for the ticklee's body to prevent any potential injury caused by the sudden and abrupt body movements which tickling can provoke. All in all, it's a very risky proposition (and position).