$product->addImageToMediaGallery($imagePath, ['thumbnail'], false, true);
$product->save();

$product->setStoreId(0)->addImageToMediaGallery($imagePath, ['thumbnail'], false, true);
$product->save();

<?php

// create ID list for products

$idList = array();
$handle = fopen("img-update-product-id-list.txt", "r");
if ($handle) {
    while (($id = fgets($handle)) !== false) {
        $idList[] = rtrim($id);
    }
    fclose($handle);
}
$idListStr = join(",", $idList);

// create PHP array representing catalog_product_entity_media_gallery_value data

$productList = array();

$servername = "localhost";
$username = "";
$password = "";
$dbname = "";
$conn = new mysqli($servername, $username, $password, $dbname);

if ($conn->connect_error) {
    die("Connection failed: ". $conn->connect_error);
}
echo  "Connection successn";

$sql1 = "SELECT * FROM `catalog_product_entity_media_gallery_value` WHERE `entity_id` IN (" . $idListStr . ")";
$result = $conn->query($sql1);
if ($result->num_rows > 0) {
    while ($row = $result->fetch_assoc()) {
        $productList[] = array(
            "value_id" => $row["value_id"],
            "store_id" => $row["store_id"],
            "entity_id" => $row["entity_id"],
            "position" => $row["position"],
            "disabled" => $row["disabled"],
            "record_id" => $row["record_id"],
            "is2nd" => false,
        );
    }
} else {
    echo "0 results.n";
}

// determine the 2nd entry for position 1 image which activates "Hide from Product Page" setting

$entityIdList = array();

foreach($productList as $key => $p) {
    if ($p['position'] == 1) {
        if (in_array($p['entity_id'], $entityIdList)) {
            $productList[$key]['is2nd'] = true;
        } else {
            $entityIdList[] = $p['entity_id'];
        }
    }
}

// update entries

$value_id = $store_id = $entity_id = $position = $disabled = "";

$sql2 = "ALTER TABLE `catalog_product_entity_media_gallery_value` DISABLE KEYS";
$conn->query($sql2);
foreach($productList as $index => $p) {

    $value_id = $p["value_id"];
    $store_id = $p["store_id"];
    $entity_id = $p["entity_id"];
    $position = $p["position"];
    $disabled = $p["disabled"];

    $is2nd = $p["is2nd"];

    if ($is2nd) {
        $sql2 = "INSERT INTO `catalog_product_entity_media_gallery_value` (`value_id`, `store_id`, `entity_id`, `label`, `position`, `disabled`) VALUES (" . $value_id . "," . $store_id . "," . $entity_id . ",''," . $position . ",1)";
    } else {
        $sql2 = "INSERT INTO `catalog_product_entity_media_gallery_value` (`value_id`, `store_id`, `entity_id`, `label`, `position`, `disabled`) VALUES (" . $value_id . "," . $store_id . "," . $entity_id . ",''," . $position . ",0)";
    }

    $conn->query($sql2);
    echo '+';  // progress indicator
}
$sql2 = "ALTER TABLE `catalog_product_entity_media_gallery_value` ENABLE KEYS";
$conn->query($sql2);

echo "n";

// remove old entries

$sql3 = "ALTER TABLE `catalog_product_entity_media_gallery_value` DISABLE KEYS";
$conn->query($sql3);
foreach($productList as $index => $p) {

    $value_id = $p["value_id"];
    $store_id = $p["store_id"];
    $entity_id = $p["entity_id"];
    $position = $p["position"];
    $disabled = $p["disabled"];

    $record_id = $p["record_id"];

    $sql3 = "DELETE FROM `catalog_product_entity_media_gallery_value` WHERE `value_id` = " . $value_id . " AND `store_id` = " . $store_id  . " AND `entity_id` = " . $entity_id . " AND `position` = " . $position . " AND `record_id` = " . $record_id;
    $conn->query($sql3);
    echo '-';  // progress indicator
}
$sql3 = "ALTER TABLE `catalog_product_entity_media_gallery_value` ENABLE KEYS";
$conn->query($sql3);

echo "n";

$conn->close();

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