#slider {
    width:940px;
    height:400px;
    display: block;
    clear: left;
    position: absolute;
    top: 0px;
    left: 50%;
    margin-left: -960px; /* change it with -(width_image_dimension/2). if your image is 1000px wide, change it with -500px. If the image is 940px wide, change it with -(940/2) = -470px */
}