What could there be on the sides of the header image? Do you want the body background to show through? For that you will need to remove the background-color of whatever element(s) have it there. If you want something else, you will have to explain more.
The gap is a combination of three different things. They can all be taken care of in your stylesheet for modern browsers, but IE6 will not recognize one of the selectors and you will need to edit tpl_header.php for it.
Add to your stylesheet
Code:
h1#indexDefaultHeading, #logoWrapper+br.clearBoth {display: none !important;}
Find this in your stylesheet
Code:
.centerColumn, #bannerOne, #bannerTwo, #bannerThree, #bannerFour, #bannerFive, #bannerSix {
padding: 0.8em;
}
and separate it and edit to look like this
Code:
.centerColumn {
padding: 0 0.8em 0.8em 0.8em;
}
#bannerOne, #bannerTwo, #bannerThree, #bannerFour, #bannerFive, #bannerSix {
padding: 0.8em;
}
Find the <br class="clearBoth" /> near the bottom of tpl_header.php and comment it out. You may have to experiment to find out exactly which one it is that is causing the gap.
There is also an empty <font> that is causing a bit of extra space above the center column main image. Look in define_main_page.php for it.