When we say traditional captcha, we refer to the out-of-focus string of words shown to users that must then be typed correctly onto a textbox to prove you are a human being and not a computer trying to game the system.
|Social captcha (work in progress)|
In my opinion, this idea may sounds great, however, I see several obstacles ahead. What happens if the photo shown is a group photo? What happens if your friend uses photos that belong to a cartoon character or someone else's e.g. an idol? Does this mean those who have added strangers as friends will have to start removing them and be more "careful" in the future?
Refreshing captcha again and again until we spot a recognized photo is going to be annoying.