See if this helps you: http://jsfiddle.net/panchroma/DK9rJ/
I've commented out some of your images so it's easier to see what's happening. The important bit is that I've added a wrapper around the area where you want the full size image background and that this wrapper is outside the div of class grid.
<div class="body-background">
<div class="grid grid-pad" id="body-wrapper-grid">
....
</div> <!-- close class="grid grid-pad" id="body-wrapper-grid" -->
</div> <!-- close body-background -->
Since the <div class="body-background">
is outside the <div class="grid ..>
it will extend full width and you will be good to go.
Good luck!