CSS for a full-width header image in Twenty Twelve:

#page {
	padding-left:0!important;
	padding-right:0!important;
	max-width:70rem;
}

header hgroup, #main, footer[role="contentinfo"], nav.main-navigation {
	padding-left:20px;
	padding-right:20px;
}

img.header-image {
	border-radius:0;
	width: 100%;
}
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