Edit in JSFiddle

<header>
</header>

body {
  margin: 0;
  background-color: #777;
}

header {
  background-image:
    linear-gradient(45deg, #777 0%, #777 calc(100% - 36px), transparent calc(100% - 36px), transparent 100%),
    linear-gradient(-45deg, #777 0%, #777 calc(100% - 36px), transparent calc(100% - 36px), transparent 100%),
    url(https://source.unsplash.com/sazU2Keuows/1000x500);
  background-repeat: no-repeat;
  background-size: 50.5% 50px, 50.5% 50px, cover;
  background-position: left bottom, right bottom, center;
  width: 100%;
  height: 250px;
}