To create from the ground up, it's actually really simple! Here is a page that actually works!
<html>
<head>
<title>ih - by balupton</title>
<script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.4.2/jquery.min.js"></script>
<style>
.ih { position:relative; }
.ih, .ih-image, .ih-info { display:block;width:330px;height:220px; }
.ih-image, .ih-info { z-index:500;position:absolute;top:0;left:0;right:0;bottom:0; }
.ih-info { z-index:501;background:black;color:white; }
</style>
</head>
<body>
<div class="ih">
<img class="ih-image" src="http://designsnack.com/screenshots/image.php?width=330&amp;height=220&amp;cropratio=330:220&amp;image=/screenshots/8264948088.jpg"/>
<div class="ih-info">
Your overlay content
</div>
</div>
<script type="text/javascript">
$(function(){
$('.ih').hover(
// Over
function(){
var $ih = $(this);
$ih.find('.ih-info').stop(true,true).animate({opacity:0.8}, 400);
},
// Out
function(){
var $ih = $(this);
$ih.find('.ih-info').stop(true,true).animate({opacity:0}, 400);
}
).find('.ih-info').css('opacity',0);
});
</script>
</body>
</html>
See it live: http://www.balupton.com/sandbox/ih.html