HTML:

<div class="img-desc">
	<img src="sample.jpg" alt="" />
	<cite>Salone del mobile Milano, April 2008 - Peeta</cite>
</div>


CSS:

.img-desc {
	position: relative;
	display: block;
	height:335px;
	width: 575px;
}
.img-desc cite {
	background: #111;
	filter:alpha(opacity=55);
	opacity:.55;
	color: #fff;
	position: absolute;
	bottom: 0;
	left: 0;
	width: 555px;
	padding: 10px;
	border-top: 1px solid #999;
}

http://www.sohtanaka.com/web-design/spice-up-your-images-with-css/

Example is here:  http://www.sohtanaka.com/web-design/examples/drop-shadow/caption.htm

Image With Transparent Caption

CSS