By which you mean crop, edit, or choose frames out of to make a shorter or longer sequence?
ffmpeg and
avconv are the same thing, but they offer a lot of options, not in editing the individual frames, but in selection and organization of frames. Further, I'm not sure they're good for letting you look at all or large groups of frames on the screen to edit that way; they're more command line tools which allow you selection flexibility.
If you're looking for something containing storyboard features, these tools aren't that.
