body {
  background-image: url(../img/Shop_Bilder/registrieren.jpg);  
  /* Background image is centered vertically and horizontally at all times */
  background-position: center center;  
  /* Background image doesn't tile */
  background-repeat: no-repeat;  
  /* Background image is fixed in the viewport so that it doesn't move when 
     the content's height is greater than the image's height */
  background-attachment: fixed;  
  /* This is what makes the background image rescale based
     on the container's size */
  background-size: cover;   

}