Apart from @DavidWachtfogel's answer, which gives good information on the difficulty of deterministically turning a face into a sequence of bits, I would like to point out that your face cannot be a "key" in the cryptographic sense, because it is not secret. You literally show it to everybody in the street, and if your face was your key, then your Facebook page just revealed it to the whole world.
Face recognition systems can be defeated by showing a printed photograph of the authorized individual in front of the camera. That attack is defeated in two ways:
Attacker's lazyness: the attacker is not after you specifically, he just stole your phone without knowing who you are, and he did not bother to take of photo of your face (which is not that hard, since mobile phones are cameras nowadays).
An armed guard who controls that what you put in front of the camera is indeed your biological face, not a substitute. This makes sense when the face recognition system is for opening a security door; not for a laptop or smartphone.
Neither applies to the specific confidentiality needs of an encryption key.