The pages look different on different computers !

I have to say I am disappointed with Squarespace. I recently switched providers, believing that Squarespace was the best option. I work on a MacBook Pro connected to an external screen (Eizo). But when I view my website from my Mac without the external screen or other types of computer, the images are incorrectly scaled. Shouldn't it automatically adjust according to the screen or computer being used ?

www.johntpedersen.com