For Themeblxd Nivo Slider:

.home #tb-slider-home:before {
	content: "";
	display: block;
	position: absolute;
	bottom: -28px;
	left: 0;
	width: 100%;
	height: 28px;
	background: url('../images/shadow.png') no-repeat;
	background-size: 100%;
}

The Image: