The pictures of the websites shown in the laptop are actual screenshots taken at viewport size of 1280 x 800. this is how it would look like on e.g. a 13" MacBook Pro.
The pictures in the mobile phones are real screenshots from an iPhone 4. you may wonder why use such a small and outdated phone to show off websites... well, if a site looks good on an iPhone 4, it will certainly look better on anything newer and larger. 320x480 is the lowest screen resolution we typically design for, hence the iPhone 4 picture.
Of course sites with responsive design will scale to any display/viewport size.