The new Facebook don’t have option to hide your display picture from strangers.
The old facebook, the popular social networking website, interface has the option to hide your display picture from people not in your friend list, but now, the new script, don’t offer any such option.
This is really an important feature required by many users, but looks like facebook stats say something different and they think that this feature was useless! maybe not much users used it in the old script so it wasn’t made part of the new script of Facebook.
What you can do is that you can hide the albums only, but not the main profile picture. It will always be shown!
Facebook is improving many aspects of it, but at the same time, changing or removing many required security and privacy features from their social networking application website!