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;
    top: 0;
    left: 0;
  
}

@media screen and (max-width: 1024px) { /* Specific to this particular image */
    img.bg {
      left: 50%;
      margin-left: -512px;   /* 50% */
    }
}