I would add it specifically for the portfolio image itself. Before, the box shadow was applied to the entire content area (image title and content space). To add it just to the image add this –
.portfolio img {
box-shadow: 0 3px 6px rgba(0, 0, 0, 0.2);
-moz-box-shadow: 0 3px 6px rgba(0, 0, 0, 0.2);
-webkit-box-shadow: 0 3px 6px rgba(0,0,0, .2);
}