I usually use a shortcut in mpv to extract the screenshot. If I want to do it via the command-line:

  ffmpeg -ss 00:00:12.435 -i '/Users/weinzieri/videofile.mp4' -vframes 1 '/Users/weinzieri/image.png'

The means “go to 00:00:12.435 on the file /Users/weinzieri/videofile.mp4 and extract one frame to the file /Users/weinzieri/image.png”.