You didn't think your 'hidden' Facebook pictures were actually, you know, 'hidden' or anything did you?
This app can find them. Sorry.
Picturebook, a browser extension now available in the Chrome web store says it can let you view hidden photos of anyone on Facebook— even if you aren't friends with them.
No app or browser extension — this one included — is able to change the privacy settings of someone else's photo. You can make it go away by selecting hide from my timeline.