Sprite Images Won't Switch Out

I’m new to SpriteKit. I started a project and I have different sizes of the sprites that I’m using. However, for some reason, when I switch between the different iPhone simulators the images won’t switch out. What I mean is, the size of the Sprites that I have for the iPhone 4s won’t change out for the larger version when I switch the simulator to iPhone 6 Plus. Is there a certain setting that I’m missing?

Thank you in advance.

are you applying the @ suffix to the name of the image ?

eg:

image1@2x.png
image1@1x.png
image1@3x.png