Hiya Mat,
That is the default behaviour of a div. What is it that you are trying to achieve? If you want the 'headerimage' div to contain an image, then you would set the background image to point to the one you want, i.e. in your screen.css stylesheet you would have:
#outerWrapper #header #headerimage {
background-color: #66CCFF;
height: 360px;
width: 820px;
background-image: url(path_to/your_image.jpg);
}
As a background image, it would not affect the positioning of any of the divs within the headerimage div.
If you wanted extra images or text to appear on top of that, you would need to add extra divs within the headerimage div - as you have with the logo div and the buckpanel divs.