Personally I think a poster is a large image piece on a medium with words and if there is no image then the type is designed as an image. The poster is not fine art but a graphic design that is concerned with a specific communication using words and image.
Like an illustration or an advertisement and unlike a painting, a poster must be superficial—what is to be communicated should be on the surface, easily understood. On the other hand a poster is not like an advertisement which has a very specific purpose and a specific call to action. Even if a poster has a call to action it is not unusual to see a movie poster or a poster for an event or play to be hanging on someone’s wall for years, often framed—it is not unreasonable to think that a poster is more than a superficial message and we can expect there to be something in the design which requires the viewer to bring something to it, like we expect in a painting.
There is a page of thumbnails (see link on left) with links to a specific image and you can navigate from one poster to another .