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