.authentication-bg {
    background-image: url('../images/whizzetlogin.png');
    background-size: cover;
    background-position: center;
    background-repeat: no-repeat;
    position: relative;
    width: 100%;
    height: 100%; /* ahora sí funciona porque el padre tiene altura */
}

.authentication-bg .bg-overlay {
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    background: rgba(0,0,0,0.45);
    z-index: 1;
}