$content_width variable

Hi All,

From my understanding of $content_width, if i do this
if ( ! isset( $content_width ) ) $content_width = 400;
on my themes functions.php and this on my themes style.css

.size-auto,
.size-full,
.size-large,
.size-medium,
.size-thumbnail {
	max-width: 100%;
	height: auto;
}

.the images and videos sizes will be reduces to 400px if they exceed,
but i have an images of 1200px by 400 px and its overflowing my content area,which is a div of 760px wide,I’m using twitter bootstrap.

is my understanding of this variable wrong or what i’m i doing wrong,please assist,

Regards.