.gallerycontainer{
position: relative;
height: 500px;
/*Add a height attribute and set to largest image's height to prevent overlaying*/
}

.thumbnail img{
border: 1px solid black;
margin: 0 5px 5px 0;
}

.thumbnail:hover{
background-color: transparent;
}

.thumbnail:hover img{
border: 1px solid white;
}

.thumbnail span{ /*CSS for enlarged image*/
position: absolute;
background-color: black;
padding: 5px;
left: -800px;
border: 1px solid #666;
visibility: hidden;
color: white;
text-decoration: none;
font-weight: bold;
}

.thumbnail span img{ /*CSS for enlarged image*/
margin-top: -35px;
border-width: 0;
padding: 2px;
}

.thumbnail:hover span{ /*CSS for enlarged image*/
visibility: visible;
top: 185px;
left: 230px; /*position where enlarged image should offset horizontally */
z-index: 50;
}