#gototopBtn {
    width: 40px;
    height: 40px;
    position: fixed;
    right: 0;
    bottom: 30px;
    background: url(../assets/images/gototop.png) no-repeat left top;
    margin-left: 610px;
}

#gototopBtn:hover {
    background: url(../assets/images/gototop.png) no-repeat left -40px;
}