.overlay {
	bottom: 25px;
	height: 60px;
	position: absolute;
	width: 30px;
	z-index: 10000;
	left: 52%;
}
 .overlay.a {
	background: url(thumbnails-a.png);
	left: 52%;
}
 .overlay.b {
	background: url(thumbnails-b.png);
	right: -10px;
}

/* CSS-based transitions: Infinite effects */