#toTop {
    display: none;
    position: fixed;
    bottom: 10px;
    right: 10px;
    width: 30px;
    height: 30px;
    background-image: url('up.png');
    background-size: 100% 100%;
    -moz-background-size: 100% 100%;
    -webkit-background-size: 100% 100%; 
    background-repeat: no-repeat;
    opacity: 0.4;
    z-index: 99999;
    filter: alpha(opacity=40); /* For IE8 and earlier */
}
#toTop:hover {
    opacity: 0.8;
    filter: alpha(opacity=80); /* For IE8 and earlier */
}