#myBtn {
    right: 50%;
    position: fixed;
    bottom: 4px;
    text-decoration: none;
    background-image: url(../files/arrow50off.png);
    width: 50px;
    height: 50px;
    background-repeat: no-repeat;
    display: none;
    padding-top: 10px;
    text-align: center;
    margin-top: 20px;
    margin-bottom: 14px;
}

#myBtn:hover {
    background-image: url(../files/arrow50on.png);
}