Egg in movie trailer and poster

The scene of Kane inspecting the egg was shot during post-production. A fiberglass egg was used so that actor John Hurt could shine his light on it and see movement inside, which was provided by Ridley Scott fluttering his hands inside the egg while wearing rubber gloves. The top of the egg opened via hydraulics, and the innards were made of a cow's stomach and tripe. Initial test shots of the eggs were filmed using hen's eggs, and this footage was used in early teaser trailers. For this reason a hen's egg was used as the primary image for the film's advertising poster, and became a lasting image for the series as a whole rather than the Alien egg that actually appears in the film.

Its more symbolic. i mean if they would use an alien egg it would 1. Spoil the real design and 2. The Public might not identify it as an egg at all. so they kept the chicken egg Design until now ;)

..........and, it was meant, with the cracking of the egg, to suggest a hatching of sorts...it gives the impression of something being born, which happens to be the most visceral scene in the film.....The chestburster scene.....