html {
    background: yellow;
    /*background-image: url('images/lxl_bg_vert.jpg');
    background-size: 100% auto;*/
}

body {
}

#main-image {
    width: 80%;
    max-width: 600px;
    margin: auto;
display: flex;
  align-items: center; /* Vertically centers child elements */
  /* If you also want horizontal centering: */
  justify-content: center;
  height: 100vh; /* Example: make parent take full viewport height */
}

#main-image img {
    width: 100%;
    height: auto;
}