how to make multiple screen captures from a video file to jpegs?

Hello,

How can I make multiple screen captures from an avi file to jpg format?
I have many 20 minute videos and I would like to make 40 screen captures from each video. I also have mpg, wmv and mov files which I want to capture 40 frames.

How can I do this? please help!

thanks
Tom

thanks, I also found this...

$ ffmpeg -i video_name.avi -r 1/20 %03d.jpg

it will extract an image from the video every 20 seconds