How did a face get tagged in a PNG file????

I used Printkey 2000 to catch a part of my screen which was on my website. Not facebook. The captured image had a facebook friend in among all the other pixels. There was nothing in that final PNG image to ID him as my FB friend. When pasted this PNG image into a post the darn thing tagged him in the photo!!! How the heck can that happen? That PNG images was just a collection of pixels with some arranged like my FB friend. I did not tag him I assure you. Is facebook using face recognition in some why they have not told us? If so that's not right!!