Entertainment Film has distribution rights for Knock Knock in the UK. They have posted a photo for the film on their FB page. I would guess that this is a promo shot that will be used in advertisements as the release date approaches in the UK since it contains comparisons to other films.
Note: As an aside, posters normally list some credits and the like at the bottom of the poster and they normally do not contain mention and comparison of other films on it.
Update: We know know that the photo backdrop forms the basis for the poster. Keanu Reeves’ billing at the top on the official poster is significantly bolder and stands out much better. The usual short list of credits and associated language is present at the bottom of the poster and all mention of other films is gone.
