#slider {
    position:relative;
    width:594px; /* Change this to your images width */
    height:400px; /* Change this to your images height */
    background:url(/nivo-slider/themes/default/loading.gif) no-repeat 50% 50%;
}
#slider img {
    position:absolute;
    top:0px;
    left:0px;
    display:none;
}
#slider a {
    border:0;
    display:block;
}