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

.thumbnail:hover{
background-color: transparent;
z-index: 0;
}

.thumbnail span{ 
position: absolute;
background-color: lightyellow;
padding: 5px;
border: 1px dashed gray;
visibility: hidden;
color: black;
text-decoration: none;
top: -30px;
left: -320px;
width: 300px;
}

.thumbnail span img{ 
border-width: 0;
padding: 2px;
top: -30px;
left: -320px;
width: 300px;
}

.thumbnail:hover span{ 
visibility: visible;
top: -30px;
left: -320px;
width: 300px;
}