Try giving your .slideshow element a position value of position:relative and the same for .modal. Then give the .slideshow element a z-index: -1 value.
Updated the fiddle: http://jsfiddle.net/BY4fu/6/
.slideshow {
height: 125px;
width: 232px;
margin-left:20px;
position: relative;
z-index: -1;
}
.modal {
z-index:1000;
background: #fff;
width: 300px;
margin: 20px auto;
border-radius: 30px;
border: 6px solid #000;
padding: 20px;
text-align:justify;
}