[ruslan_mart]

#header {
  display: block;
  height: 273px;
  background:url('..//images/ng_vykl.jpg') no-repeat center bottom;
}
#header.bg2 {
  background-image: url('..//images/ng_vykl-2.jpg');
}

<button onclick="header.classList.toggle('bg2')">Button</button>