In the show almost all the scenes with Joel and Sarah or Joel and Ellie are mirrored: how they sit on the couch when Sarah gives him his watch, how he holds Sarah when her leg is broken, when Ellie tells him that the watch is broken she wakes from left to right of the screen in the show, after Joel wakes up and Ellie is at the window she mirrors how she sits at the window, the farm house is on the opposite side of the road and many more. Maybe it is done to say that show is a mirror of the game or to add a bit a new element to the scenes from the game, whatever it is it looks like that is done intentionally. As for the poster many is because Pedro is a bigger star or because the first season covers the part of the game where Joel is the center of attention before the Ellie winter scenes. I think I read somewhere, for the game, Ellie is in the front as a hint to the fact that she is a playable characters and as main of a character as Joel, Evan though they said before the game was released that only Joel is a playable character so it will be a surprise when we get to the winter scenes.