Basicly, This is what i’m trying to get.
Do you mean the picture of the tricycle? That’s not actually a part of their header. The header is just the site title and navigation (it’s the part that turns pink when you scroll down). The photo is part of the main page, in a fixed position behind the header.
In any case, if you want the photo to scale proportionally, the easiest approach would be to use an actual
img element and just set the width to 100%.