[FFmpeg-user] Image extraction broken with ffmpeg, but not with mencoder. How can I fix this?

[James Board]

>If input frame rate is smaller than output frame rate, ffmpeg will create

>extra frames by default.
>
>If input is vfr and output is cfr than it may duplicate and/or remove frames.

Everyone keeps mentioning vfr and cfr.  I know what vfr and cfr mean but
I don't see how that has anything to do with my issue, because I'm not
creating a video as output.  I'm only creating images for every frame in
the input.  So, can someone explain why this matters?  

I understand the fact that if the output is cfr and the input is vfr, then 
some frames may need to be duplicated or dropped.  But if the output is 
merely a sequence of images, why does this matter?  There's no frame rate 
in a sequence of images,