Hi  @pg,
For responsive image, you’ll need modify background-size property in the above code. Change the code background-size : cover to background-size: contain so it would look like following:
#promotion-message .section {
	background-image   : url("http://mattfurniture.co.uk/wp-content/uploads/2016/02/matt-furniture-logo-rex.png")  !important;
	background-position: center center;
	background-repeat  : no-repeat !important;
	background-size    : contain;
}
With this change, the background image will be a bit smaller than what it is now, and it will be responsive.
Regards,
Mahesh