For what it's worth, a former film professor at BU told me a story once about
how he had found it difficult to impossible to get permission to use actual
film frames to illustrate a more-or-less scholarly book. He said that the
movie studios want everyone to use "production stills" instead, which are
high-quality conventional still photographs taken more or less concurrently
with the actual production. I'm not sure exactly why they want this, but
apparently most photographs "from films" that appear in traditional print
media are production stills rather than actual frames.
In his case, it was very annoying because he wanted to point out some
lighting and compositional details which were quite different in the
production stills and the actual film.
He eventually shrugged his shoulders and decided to risk using the frame
grabs without permission. This was a while ago and he never received any
lawyer letters about it.
His belief system--and I regard him as knowledgeable about this--was that
_nobody really knows_ the the legal situation in such matters, absent an
actual lawsuit. He held the conspiracy theory that intellectual properly laws
are deliberately vague and ambiguous, because that insures power remains in
the hands of companies big enough to retain lawyers.