Your max-width on your #page is set to 1000px, so you'd have to change that (it's too small).
However, the body is set to have padding on either side, I suspect so that the pink color under there shows through. So you'll have to set your body to have 0 padding, too.
Then take your #main attribute and set it back to max-width 1000px (so it's not taking up your whole page), and set the margins on #main so:
Then you'll have to go into #branding img and I would re-upload an image that is the right size. If you set the width to 100%, you could upload an image with a generous white space to the left and right to ensure that you don't stretch the image to fit. By setting the width to 100%, you get the border all the way across the bottom.
I hope that's helpful!