function getPictureFqn(string $pictureType): ?string
{
    $fqn = 'App\\Entity\\' . $pictureType;
    if ($fqn === Picture::class) {
        return Picture::class;
    }
    $refl = new \ReflectionClass($fqn);
    if (!$refl->isSubclassOf(Picture::class)) {
        return null;
    }

    return $fqn;
}

    /** @var class-string<Picture> $fqn */
    $fqn = 'App\\Entity\\' . $pictureType;

    /** @var class-string<Picture|object> $fqn */
    $fqn = 'App\\Entity\\' . $pictureType;

class Picture
{
}

class PictureUser extends Picture
{
}

class PictureProduct extends Picture
{
}