Thanks again for your help, Ralph. As you can see, I'm a begginer and I'm having some trouble with this header. When I replace width in pixels for 90%, there's no way to align header and menu n the center of the page and, as you've mentioned, I still have the big background image.
Since the beggining, I wanted to have the white background associated to the main column and not as a background image, but didn't know how to do that. Is that really a solution? If so, how can it do that?