credo che sia esattamente il prototipo, per così dire, della locandina del film, a partire dal quale viene poi creato tutto il restante materiale pubblicitario. non ho trovato una traduzione più specifica.

qui spiegano esattamente che cos'è e la procedura cui ho accennato:

Once the look of the poster is established it becomes what is called the "key art". The key art is then utilized by other designers, graphic studios or ad agencies to lay out the other forms of advertising. Standees might be designed by the same art director or design studio that designed the poster, but it would then be built and produced by companies that specifically make displays and standees.

from: www.hollywoodlitsales.com/archives/barbara.shtml

vedi anche, solo per esempio:

SevenQuestions: Jon C. Allen - [ Traduci questa pagina ]
... of a film advertising campaign are built around the "key art" (the movie poster). ... You might see the movie poster's film title logo used in trailers. ...
www.sevenquestions.com/new7q/jonallen.shtml