Digital cameras sensors all have unique imperfections that can be used to fingerprint an image as coming from the same camera. If you are cropping your face from an image to avoid being tied to your real name or other online accounts, you should be aware of this.
More info (and mitigation strategy ideas):
https://33bits.org/2011/09/19/digital-camera-fingerprinting/
http://dde.binghamton.edu/download/camera_fingerprint/