I’m trying to do something like this:

The boxes have shadows and the background of the corners must be transparent because they are over an image (unpredictable background).
After searching Google, I found solutions using pseudo selectors :before and :after as well as solutions using extra markup, but all of them use a fixed colour background. These were my results:

I’m trying to use box-shadows and only a small image for the corner, instead of a large complete background.
How I can do this?
Use both the pseudo-elements, one for the upper box, the other for the white triangle:
Here’s the fiddle: http://jsfiddle.net/Kjp6v/
This does not add a shadow under the fold, but looks realistic enough.