First head over here to download FFMPEG. if you happen to be a Mac user, you can use Homebrew:
brew install ffmpeg
I would check out the Homebrew page for all of the options available (of which, there are quite a few).
To extract frames from a video file, you use the following command:
ffmpeg -i my_vid.webm -r 10 -frames:v 30 -f image2 foo-%03d.jpeg
The above command extracts 10 frames per second and saves them as jpeg files with names ‘foo-001.jpeg’, ‘foo-002.jpeg’, and so on. The maximum number of images that will be created is 30.
image2 refers to the demuxer used.
You can read more about this in the documentation.