I think it’s interesting to compare these two posters for Teeth. The one above is new and is part of the promotion for the upcoming theatrical release. The one below was used when the movie was in the festival circuit.
Like Chris says, the new poster positions the movie as a clear cut sexual thriller, even going to the extent of using a critic quote that mentions Fatal Attraction. The image does a good job of being alluring but still maintaining a sense of danger.
The old poster positions the movie as more of a comedy. Certainly a quirky comedy, and perhaps one with horror elements, like Shaun of the Dead, but definitely a comedy.
If the name wasn’t the same it would be hard to tell that both posters are for the same movie.
As I understand, the actual movie has both the more horror/thriller elements and the comedy side. Balancing those elements in a movie is very hard. Balancing then in a poster is almost impossible. The marketing team seems to have decided that it would be more effective to sell just the more serious sexual thriller aspect in the poster. Can’t say I disagree with them.