Honestly, I think I could come up with a better cover with a few hours on Photoshop. The added graininess of those pictures doesn't work for me either. What would be superior is to use those shots for a character reference, but have an artist draw them like an old movie poster.
These pictures aren't the best ones they took, IMO. They thought the angle on the Kirk one made him look 'cool', but it really doesn't. Leave that downward head tilt to a villain or someone who is genuinely a bit edgier. Kirk would do better with a generic hero shot. Also, that shot naturally minimizes the chin, which isn't smart on a person who doesn't have a strong chin (unless you're talking about goth doll shots). You need powerful, large eyes to pull that angle off effectively. That's just not a great example of pictures done at that angle. And I'm surprised they have dropped Spock right off of the cover when the movie went out of its way to make it a duo piece.
Kirk, Spock, Uhura, McCoy and Spock Prime are the characters who make the most sense to be given cover treatment.