Here's how it works: When you post pics, only people who are your friends can see them. However, if I am friends with "Jane" and Jane comments on "Sam's" picture, this comment (and Sam's picture) get posted for ME to see. And I've never met Sam in my life. Not only that, but I can click on the picture and see the whole album of all Sam's pictures. Just the one album though. So, poor Sam is getting his pictures looked at by every single person who is Jane's friends. And, the same goes for anyone else who comments on his picture.
Imagine that, all your friends friends can look through your entire album every time that friend comments on a picture. So until Facebook closes this privacy loophole you might want to protect your friends privacy, by not commenting on pictures. Imagine what parents, coworkers, and others think when they see pictures of you that you don't want them to see!