var lastBanner 		= 13;
var randomBanner	= Math.floor(Math.random()* lastBanner) + 1;
var thisBanner		= randomBanner;

function nextBanner(hint) {

		if (hint > 0) {
			thisBanner++;
		} else {
			thisBanner--;
		}

		if (thisBanner <= 0) {
			thisBanner = lastBanner;
		}

		if (thisBanner > lastBanner) {
			thisBanner = 1;
		}

        newImage = "url('_images/masthead-" + thisBanner + ".jpg')";
        document.getElementById('container').style.backgroundImage = newImage;

}