img.bg {
  /* Set rules to fill background */
  min-height: 100%;
  min-width: 1024px;
  /* Set up proportionate scaling */
  width: 100%;
  height: auto;
  /* Set up positioning */
  position: fixed;
  /* またはabsolute; */
  top: 0;
  left: 0;
  z-index:-9999;
}


@media screen and (max-width: 1024px){
  img.bg {
  z-index:-9999;
  left: 50%;
  margin-left: -512px; }
}
 
div#container {
  position: relative;
}