The artist used several pictures of each dress from different angles, and had to research references in articles for the older ones, as the photos were in black and white. The article (read it here) also states the difficulty they had finding designers of the older dresses, which is why many of them are marked unknown. I really like how they didn't include the actresses themselves, and just the dress. It enables you to use your imagination a little bit. So many of these dresses are iconic to the point that when you think of the actress, you think of her in that dress. Very neat idea for a poster.
(photo collected from mediarundigital)