img.bg {
	/* Set rules to fill background */
	min-height: 100%;
	min-width: 792px;

	/* Set up proportionate scaling */
	width: 792px;
	height: auto;

	/* Set up positioning */
	position: fixed;
	top: 0;
	left: 0;
}

@media screen and (max-width: 792) { /* Specific to this particular image */
	img.bg {
		left: 50%;
		margin-left: -512px;   /* 50% */
	}
}