Dataset of faces: https://www.dropbox.com/s/upqoe5lytacqowl/faces.zip?dl=0

Following the hint, we use an off the shelf face detection python scriptto look for images that do not have any faces detected.

#!/bin/bash
for i in `seq 1 12070`;
do
    python3 ./FaceDetect/face_detect_cv3.py ./reduced_size/"$i".jpg
done

And we find:

../reduced_size/1145.jpg: Found 0 faces!

Open the image to find the flag at the bottom of the image: