Edit in JSFiddle

<!DOCTYPE html>
<html lang="it">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Landing Page</title>
    <link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/4.1.0/css/bootstrap.min.css">
    <style>
        .sfondo {
            background: url('https://loremflickr.com/320/240?random=1') no-repeat center center;
            background-size: cover;
            height: 100vh;
        }
        .cta {
            position: relative;
            top: 35%;
            transform: translateY(-35%);
            background: rgba(0, 0, 0, 0.5);
            padding: 5rem; 
        }
    </style>
</head>

<body>
    <div class="sfondo">
        <div class="cta container rounded text-center text-white">
            <h1>Ricevi 4 Video Lezioni Gratuite</h1>
            <p class="lead p-3">
                Sei ad un passo dalla svolta. Bastano pochi minuti.
            </p>
        </div>
    </div>
</body>

</html>