Strange enough, but there is only a div on your page, no <html> <head> css and so on.
Try something like this
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>Gallery</title>
<style type="text/css">
/*Credits: Dynamic Drive CSS Library */
/*URL: http://www.dynamicdrive.com/style/ */
.gallerycontainer{
position: relative;
/*Add a height attribute and set to largest image's height to prevent overlaying*/
}
.thumbnail img{
border: 1px solid white;
margin: 0 5px 5px 0;
}
.thumbnail:hover{
background-color: transparent;
}
.thumbnail:hover img{
border: 1px solid blue;
}
.thumbnail span{ /*CSS for enlarged image*/
position: absolute;
background-color: lightyellow;
padding: 5px;
left: -1000px;
border: 1px dashed gray;
visibility: hidden;
color: black;
text-decoration: none;
}
.thumbnail span img{ /*CSS for enlarged image*/
border-width: 0;
padding: 2px;
}
.thumbnail:hover span{ /*CSS for enlarged image*/
visibility: visible;
top: 0;
left: 230px; /*position where enlarged image should offset horizontally */
z-index: 50;
}
</style>
</head>
<body>
<div class="gallerycontainer">
<a class="thumbnail" href="#thumb"><img src="images/Thumb_ESGChtA.jpg" width="100px" height="66px" border="0" /><span><img src="images/ESGChtA.jpg" /><br />ChartA.</span></a>
<a class="thumbnail" href="#thumb"><img src="images/Thumb_ESGChtA.jpg" width="100px" height="66px" border="0" /><span><img src="images/ESGChtA.jpg" /><br />ChartA</span></a>
<br />
</div>
</body>
</html>
and see if that works