Extract key frame from video with ffmpeg
No ffmpeg code can identify interesting frames. At most, there are filters which detect representative frames and filters which identify frames with a scene change. So, at best, there's a thumbnail filter for this, sort of. It can't detect "interesting" frames but it detects representative frames. So, frames from the middle of fades should be out. 

Syntax is

ffmpeg -ss 50 -i in.avi -vf thumbnail=300 -vsync 0 thumbs%d.png


This will pick one representative frame from each batch of 300 consecutive frames.