Tom Hanks is in this movie — although in the end, it was Michael Clarke Duncan who got the acting accolades — and he was arguably the most famous person in the cast, so I get making him the face of the film. He’s Tom Hanks. He’d won two Oscars at this point and was nominated for two more (did you know he scored a nod for Big?), including for Saving Private Ryan the year before. So yeah. He’s Tom Hanks, he was Tom Hanks, he will always be Tom Hanks, and for a lot of people the simple act of slapping Tom Hanks’s face on a poster will be enough to get their butts in seats.
But… then why not just use Tom Hanks’s actual face? This is not a photo of Tom Hanks. This is an artist’s recreation of what would happen if you melted down Tom Hanks and then tried to rebuild him from memory — but you’d recently banged your head and so everything was a little shaky, including your manual dexterity. They would have been better off replacing his face with the words, “THIS GUY DIRECTED SHAWSHANK REDEMPTION, SO MAYBE THIS MOVIE IS JUST AS GOOD,” and crossing their fingers.