.b-up_button {
    position: fixed;
    bottom: 60px;
    left: 50%;
    margin-left: -560px;
    width: 39px;
    height: 40px;
    display: none;
    z-index: 100;
}

.b-up_button span {
    display: block;
    height: 100%; 
    background: url('img/up.jpg') no-repeat 0px 0px; 
    opacity: 0.3;
    filter: alpha(opacity=30);
}