Forum Moderators: not2easy
#header { width: 900px; margin: auto; }
for whatever your header width is, or
#header { text-align:center; }
Even if it's an image and not "text".
It's always helpful in any problems with positioning to put a border on the element temporarily to see what's happening:
#header { width: 900px; margin: auto; border: 1px solid #ff0000; }
#header img { border: 1px solid #00ff00; }
You'll see by default the div will be the full width of any element containing it.