html {
  background: url(backgroundimage.jpg) no-repeat center center fixed;
  -webkit-background-size: cover;
  -moz-background-size: cover;
  -o-background-size: cover;
  background-size: cover;
  font-size: 2.5em;
  color: white;
}

.centered {
  position: fixed;
  top: 25%;
  left: 50%;
  /* bring your own prefixes */
  transform: translate(-50%, -50%);
}


.my-container {
    position: relative;
    background: #5C97FF;
    overflow: hidden;
}
/* You could use :after - it doesn't really matter */
.my-container:before {
    content: ' ';
    display: block;
    position: absolute;
    left: 0;
    top: 0;
    width: 100%;
    height: 100%;
    z-index: 1;
    opacity: 0.6;
    background-image: url('backgroundimage.jpg');
    background-repeat: no-repeat;
    background-position: 50% 0;
    -ms-background-size: cover;
    -o-background-size: cover;
    -moz-background-size: cover;
    -webkit-background-size: cover;
    background-size: cover;
}