epatt250 has some good points. In the original there's not much hot spots and not only is the skin super smooth on everyone, but their hair, clothes, everything has almost a digital painting feel to it. Also I'm pretty sure they did it on purpose all the characters are super pale except the human girl, she still has a tint of pink in her flesh.
But yeah like most people said, try overlapping the pictures and it might add to it a bit. Also maybe bring the couple up a little larger than the rest of the people. It looks like you did that already, but maybe just a bit more so they're the main focus and the rest are just supporting cast. If it was a movie poster you would think the couple is obviously the most important just because they're the only couple and they're in the center, but you would also assume the movie would have a lot of screen-time for the other 3.
Another thing I just noticed, have no idea if this was intentional or just happened. On the official Twilight poster, all three characters on the left of the poster are looking off to the right side. While all three characters on the right of the poster are looking into the camera or at least towards the camera. Kind of interesting, don't think it has any real effect on the poster but never know.




