Thats only half the story so far....
The background-inage is fixed but the semi transdparency is created using filters.
Until the release of NS6.1 there were no major browsers other than IE supporting filters but now the new -moz filters are available for ns6 too.
The code used onthe site you highlited is simply
.hilite {filter:light();}
.flipHilite {filter

rogid

XImageTransform.Microsoft.BasicImage(grayscale=0,xray=0,mirror=1,invert=0,opacity=1,rotation=0);}
these classes are then appplied to the layers you wish to have a effect on. Filters can create all sorts of effects but opacity chanegs creating a semi transparent feel is the most common.
The simplest way to do this is
.fade{filter:alpha(opacity=50,enabled=1);
and for ns i think you wouild add
-moz-opacity:0.7
yet again a discrepancy between the big two but since there are currently no standards supporting the use of filters thats not sobad.
for more info checkout
its fairly simple to follow.
hope this helps
rob