ok, a friend of mine is bugging me for help about a movie from his youth.
He didn't actually see the movie, just the "coming soon" poster. It was around 1981 or '82, and the poster was just a woman's head beheaded, with a black background.
Not much to go on, but...?
I think it's a movie called 'Dark Sanity'.
In England it was released on video around 1983, on the VIPCO label, and the cover looks just like your description.
Not seen the film though..