I ran a quick a test and you can see from the screen shot that the pages were created symmetrical.
I do agree with you in that replacing the header the image should not effect how the template is displayed, and that is the case for most of our templates, but some of them requires that the header image to be sliced in a way to get the result we are looking for.
After you add the image to your header div, you might have to adjust the padding and height on the header class as well to position the image correctly.
