Why are there movie stills that don't appear in movies at all?

I notice that when I do image search for a movie, there will be at least one still that doesn’t appear in the movie itself. Some of those stills go on to be famous or the iconic features of the movie despite never appearing anywhere in the movie. An example is this still from Out of the Past. This is the most famous still from the movie despite never being seen in the movie.

Where do those stills come from then? Did the actors just pose for those stills for no reason?