.clase {
    position: relative;
    overflow: hidden;  
}

.clase h1 {
    position: relative;
    z-index: 1;
    opacity: 1;
}

.clase img {
    position: relative;
    z-index: 1;
    opacity: 1;
}

.clase:before {
    content: ' ';
    position: absolute;
    width: 100%;
    height: 100%;
    z-index: 2;
    opacity: 0.25;
    background-image: url('../img/fondo.jpg');
    background-size: cover;
}