#btn{
    width: 40px;
    height: 40px;
    display: none;
    position: fixed;
    left: 45%;
    bottom: 40px;
    margin-left: 610px;
    background: url("https://images.cnblogs.com/cnblogs_com/littleseasalt/2027694/o_2109080800125-120601155S9-50.gif" )no-repeat left top;
}
#btn:hover{
    background: url("https://images.cnblogs.com/cnblogs_com/littleseasalt/2027694/o_2109080800125-120601155S9-50.gif" )no-repeat left -40px;
}