#slideshow {
    position: absolute;
    top:0px;
    left:0px;
    width: 627px;
    height: 273px;
    overflow: hidden;
    z-index:-1;
}
#slideshow img {
    position: absolute;
    display:none;
    left: 0;
    top: 0;
}

.imagem_ambiente {
    width:627px;
    height:273px;
    position:relative;
    float:right;
    z-index:1;
    overflow:hidden;
}
.mask {
    width:627px;
    height:546px;
    position:absolute;
    top:0px;
    left:0;
    z-index:-1;
}