Single Frame

ffmpeg -i video.mp4 -ss 01:23:45 -vframes 1 image.png

Whole Video

Take a image every 20 seconds in the video

ffmpeg -i video.mp4 -r 0.05 image-_%04d.png

For every 10 seconds, change -r 0.1, for every 4 seconds -r 0.25

If you are going to have more than 10K images, change %04 to %05