On Superhighways they're using the Background Begin and Background End includelets to put an image onto the page, which has the option to go to the full width of the browser.

The advantage (or disadvantage depending on how you look at it) of this is that you have to set it up for each page that you want it - so each page can have a different big image, but it's not there on every page as it's not part of the theme.

If you want a big image at the top of every page then you're best bet is to try the Bootstrap theme and use the banner options to set up a top image.

Thanks

Joe