Crop a background photo when minimalized

Today, surfing on the internet I found this site (http://ift.tt/2f6Tllu). I think he did it within a software, not coded it... but it doesn't matter. I can't understand how to do that thing. When I change the dimension of the page, the background photo (where there's the word 'Welcome') is like cropped. Maybe you didn't understand. Ask me