body, html {
  height: 100%;
}
.bg-img { 
  background: url(../img/marttin-fisher-bg.jpg) no-repeat center center fixed; 
  -webkit-background-size: cover;
  -moz-background-size: cover;
  -o-background-size: cover;
  background-size: cover;
  bottom: 0px;
  top: 0px;
  width: 100%;
  height: 100%; 
}
.centered-content{
	width: 500px;
	max-width: 100%;
	text-align: center;
	margin: 30% auto;
}

@media screen and (max-width: 767px) { /* Specific to this particular image */
  .centered-content {
    margin: 50% auto;
  }
}