penso che il problema si position: fixed; prova a centrare il tutto con flex.
codice:
<!DOCTYPE html>
<html lang="it">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>page test</title>
    <style>
        html,
        body {
            height: 100%;
            padding: 0;
            margin: 0;
        }

        .contenitor {
            display: flex;
            align-items: center;
            justify-content: center;
            height: 100%;
        }

        .contenitor:before {
            content: "";
            position: absolute;
            z-index: -1;
            top: 0;
            bottom: 0;
            left: 0;
            right: 0;
            background: url('../../public/sfondo_home.png') no-repeat center;
            opacity: .2;
        }

        .bg {
            width: 100%;
            text-align: center;
        }

        .myimg {
            border: 0;
            max-width: 350px;
        }

        .ebutton {
            padding: 20px 0;
        }
    </style>
</head>

<body>
    <div class="contenitor">
        <div class="bg">
            <img class="myimg" src="IMGLOGO.jpg" alt="IMGLOGO">
            <p>nome cognome</p>
            <img class="myimg" src="IMMAGINE.JPG" alt="IMMAGINE">
            <form class="ebutton" method="POST" action="menu.asp">
                <button type="submit">Menù</button>
            </form>
        </div>
    </div>

</body>

</html>