.thumbnail{
position: relative;
z-index: 0;
}

.thumbnail:hover{
background-color: transparent;
z-index: 50;
}

.thumbnail span{ /*CSS for enlarged image*/
position: absolute;
background-color: lightyellow;
padding: 5px;
left: -1000px;
border: 1px white;
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 on hover*/
visibility: visible;
width: 620px;
font-weight: normal;
top: 600px;
left: 500px; /*position where enlarged image should offset horizontally */

}

<a class="thumbnail" href="#thumb"><img src="media/tree_thumb.jpg" width="100px" height="66px" border="0" /><span><img src="media/tree.jpg" /><br />Simply beautiful.</span></a>