Since you are able to filter to only those that are clear portrait photos, I'm assuming you have some method of feature generation to transform the raw images into features that are useful for machine learning purposes. If that's true, you could try to train a classification algorithm (there are lots of them: neural networks, etc.) by feeding the algorithm a bunch of known Bieber photos as well as a bunch of known non-Biebers. Once you have trained the model, it could be used to predict whether a new image is Bieber or not.
This sort of supervised learning technique does require you to have data where you know the correct answer (Bieber or not), but those could probably be found from a Google image search. It also requires that you have the right sorts of features, and I don't know enough about image processing or your algorithm to know if that is a major drawback.