Here is mine.

To screen capture every 600 frames:

  ffmpeg -i my_vid.mp4 -vf "select=not(mod(n\,600))" -vsync vfr -q:v 15 img_%03d.jpg

To make a montage that's 1024 px wide:

  for d in \*
  do
      (montage -mode concatenate -tile 4x -resize 1024x $d"/\* "$d".jpg)
  done