#eye{
    opacity: 0;
    position: absolute;
}

#eye + label{
    display: block;
    background-image: url(../image/eye.png);
    background-size: 30px;
    width: 30px;
    height: 30px;
    position: absolute;
    top: 2em;
    right: 0.5em;
}

#eye:checked + label{
    display: block;
    background-image: url(../image/eye2.png);
    background-size: 30px;
    width: 30px;
    height: 30px;
}