Just create a div place it below with a bit of javascript
e.g
<div id='back' style="position:absolute; visibility
dden; background-color:#000000; opacity:0.4;filter:alpha(opacity=40);-moz-opacity:0.5;-khtml-opacity: 0.5;" ></div>
function setBack()
{
var opBack = document.getElementById('back');
opBack.style.visibility = 'visible';
opBack.style.top = 0;
opBack.style.left = 0;
opBack.style.width = '100%';
opBack.style.height = '100%';
}
then wack your pic in another div ontop/ cnetered