Of the two Ponyo posters from Japan, this one is my favorite. It's a direct shot from the movie, instead of a stylized reenactment, which has been the Ghibli standard for a number of years now. I suspect the home video/dvd market plays a hand in choosing the design, since these poster will appear later on the DVD release. But it remains a terrific poster, thanks to the brilliant skills of the Ghibli artists.
There is a terrific variety in texture and color in this poster, and you can admire Ponyo's unique look, that illustrated storybook style that Ghibli has danced around and played with on various shorts and features - Isao Takahata's My Neighbors the Yamadas, Yoshiyuki Momose's Ghiblies Episode 2, and the series of House Foods commercials directed by Momose and Hayao Miyazaki.
This is a very effective movie poster because, not only is it visually attractive, it establishes the central drama of the story and introduces us into Ponyo's world. The Japanese text, "umarete kite yokatta," which translates to, "I'm glad I was born" (or more loosely, I guess, "happy birthday"), reveals to us the movie's theme, aimed for young children. Ponyo is Miyazaki's first true "children's" movie in a very long time.