Code:
ffmpeg -i video.avi -r RATE -an -vcodec mjpeg -sameq -f image2 -y %08d.jpg
RATE
= 1 for every 1 second
= 2 for every 1/2 second
= 4 for every 1/4 second
= 1/2 for every 2 seconds
= 1/5 for every 5 seconds
In the case where RATE = 1/X, X cannot be > 20 or it dies.