Kraellin- When you mulitply the background layer it makes the whole image dark, but you only want the edges dark to make it a uniform exposure - by deleting (hit the delete key- or the nonviolent method, press the delete key..lol) the selection you made you delete the dark inside the box leaving only the edges darker. It will look funny because the inside box will be lighter so you will have to adjust the opacity to match the original photo.
I suppose you could mask it if you wanted to, I just delete the excess dark area- faster for me.
Does that make sense now? I need more coffee, it's too early in the morning! I will check my post to make sure I wrote it down right, sometimes I get in a hurry...
~Amber~
Okay-Yep it was me. My short and simple answer was way to short and simple for anyone to understand! Forgive me?
Open photo-
Duplicate background layer
Change Layer mode to Multiply
Now here's the key part- Take the lasso tool and feather it at about 50, now make your box around where the lighter areas are, but not too close-see diagram.
Delete the selected area, leaving the middle lighter and a dark frame around the edges.
Change opacity of layer to blend into the bottom layer. Repeat as needed.