Charlie, the stills are every bit as much of advertising as the posters are maybe even more so.
for instance, when I went to theatres as a kid, they had a whole assortment of images displayed from billboards to small formats AND large displays of stills - sometimes in proper scene order - in the outside port cochere. Then the long corridors had them also, in various sizes from 8x10 to 14x20 or even larger.
The posters are idealized images of the film story, or today, mostly just portraits. The stills are 100% representative of what you see on-screen unlike any other piece of advertising.
Finally, when yu opened up newspapers to the movie section, I don't ever recall seeing a reproduction of a poster used for a review, or one of those "here is a photo of Dick Powell in Murder My Sweet at the Pantages Theatre through Friday". Only stills were used.
Stills are great and I have thousands of them