Twenty Fifteen – Remove margins for initial image

In Twenty Fifteen I want my initial image to have no top margin and no left or right margin, as in the Twenty Fifteen publicity. Please help!
My web page shows the image which I would like to stretch the full width of the content area, below that is an example of the effect I want, below that is a screenshot of the Twenty Fifteen publicity.

The page I need help with: [log in to see the link]