media/ffmpeg/ ExtractingKeyFrames

In mp4, there are multiple types of frames. An I-frame is an entire frame: other types record changes relative to neighbouring frames.

See here:

ffmpeg -i 2.flv -vf "select=eq(pict_type\,I),scale=73x41" -vsync vfr -qscale:v 2 thumbnails-%02d.jpeg

Getting time offset of I-frames. See here

ffmpeg -show_frames input.mkv