The jQuery solution: Wrap the thumbnails in a trigger div, which is absolutely positioned over the image. Target that to fade the elements in and out.
For a CSS3 solution, refer to Vigrond's answer.
HTML
<div id="wrapper">
<img src="http://lorempixum.com/600/600" />
<div id="trigger">
<div id="thumbnails">
<img src="http://lorempixum.com/60/60" />
<img src="http://lorempixum.com/60/60" />
<img src="http://lorempixum.com/60/60" />
<img src="http://lorempixum.com/60/60" />
</div>
</div>
</div>
CSS
#wrapper { position:relative; }
#trigger {
width:100%;
height:80px;
position:absolute;
left:0;
bottom:20px; }
#thumbnails {
width:100%;
height:80px;
display:none; }
#thumbnails img {
margin:10px;
float:left; }
jQuery
$(document).ready(function(){
$("#trigger").hover(function () {
$(this).children("div").fadeTo(200, 1);
}, function(){
$(this).children("div").fadeOut(200);
});
});
See my fiddle:
http://jsfiddle.net/TheNix/Cjmr6/