.bg {
    /* Set rules to fill background */
    min-height: 100%;
    min-width: 1200px; /* Specific to this particular image */
    /* Set up proportionate scaling */
    width: 100%;
    height: auto;
    /* Set up positioning */
    position: fixed;
    top: 0;
    left: 0;
}

@media screen and (max-width: 1200px) {
    /* Specific to this particular image */
    .bg {
        left: 50%;
        margin-left: -600px; /* 50% */
    }
}