A good movie poster is a rare thing these days. Usually they end up with horrible artwork and showcases who is in it rather than leave an memorable impression on what to expect from the movie.
This one jumps out right at you. The 'Boogie Nights' font and the use of the multiple characters inside of the star give you an idea of what you are in store for.
Out of the three posters I have seen for Magnolia, this is the one that best represents the movie. What makes this poster a rarity is that it's a movie with Tom Cruise where he isn't even on the poster! Though I would usually prefer him not to be on it, in this case it would make sense for him to be there. This is a character driven drama and the subtlety of the images among the flower makes it a haunting image.
The frogs poster is great but someone who hasn't seen the movie and sees the poster will have frogs in the back of their mind when actually watching the film. The other one I have seen is a cool mosaic of the charcters, but the reviews of the film that are present on that poster clutter up the spaces of it.