Latest Guide on How to Add Images To Product in Magento 2

<?php
ini_set('display_errors', 1);
ini_set('display_startup_errors', 1);
error_reporting(E_ALL);
 
/*
 * Assumes doc root is set to ROOT/pub
 */
require_once dirname(__DIR__) . '/app/bootstrap.php';
$bootstrap = \Magento\Framework\App\Bootstrap::create(BP, $_SERVER);
 
class AssignImages extends \Magento\Framework\App\Http implements \Magento\Framework\AppInterface
{
    public function launch()
    {
        $state = $this->_objectManager->get('Magento\Framework\App\State');
        $state->setAreaCode('adminhtml');
 
        $galleryReadHandler = $this->_objectManager->create('Magento\Catalog\Model\Product\Gallery\ReadHandler');
        $imageProcessor = $this->_objectManager->create('Magento\Catalog\Model\Product\Gallery\Processor');
        $productGallery = $this->_objectManager->create('Magento\Catalog\Model\ResourceModel\Product\Gallery');
 
        /**
         * Assumed images are named [sku].[ext] and reside in ROOT/pub/media/[image_dir]
         */
        foreach (glob(__DIR__ . "/media/[image_dir]/*.{jpg,png,gif}", GLOB_BRACE) as $image) {
            $imageFileName = trim(pathinfo($image)['filename']);
            $sku = $imageFileName;
            try {
                $product = $this->_objectManager->create('Magento\Catalog\Model\Product')->loadByAttribute('sku', $sku);
                if ($product) {
                    $galleryReadHandler->execute($product);
 
                    // Unset existing images
                    $images = $product->getMediaGalleryImages();
                    foreach($images as $child) {
                        $productGallery->deleteGallery($child->getValueId());
                        $imageProcessor->removeImage($product, $child->getFile());
                    }
 
                    /**
                     * Add image. Image directory must be in ROOT/pub/media for addImageToMediaGallery() method to work
                     */
                    $product->addImageToMediaGallery('[image_dir]' . DIRECTORY_SEPARATOR . pathinfo($image)['basename'], array('image', 'small_image', 'thumbnail'), false, false);
                    $product->save();
                    echo "Added media image for {$sku}" . "\n";
                }
            } catch (\Exception $e) {
                echo $e->getMessage();
            }
        }
        return $this->_response;
    }
 
    public function catchException(\Magento\Framework\App\Bootstrap $bootstrap, \Exception $exception)
    {
        echo $exception->getMessage();
        return false;
    }
}
 
/** @var \Magento\Framework\App\Http $app */
$app = $bootstrap->createApplication('AssignImages');
$bootstrap->run($app);

// Instance of object manager
$objectManager = \Magento\Framework\App\ObjectManager::getInstance(); 
//Remove Images From Product
$productId =  1 ; // Id of product
$product = $objectManager->create('Magento\Catalog\Model\Product')->load($productId);
$productRepository = $objectManager->create('Magento\Catalog\Api\ProductRepositoryInterface');
$existingMediaGalleryEntries = $product->getMediaGalleryEntries();
foreach ($existingMediaGalleryEntries as $key => $entry) {
    unset($existingMediaGalleryEntries[$key]);
}
$product->setMediaGalleryEntries($existingMediaGalleryEntries);
$productRepository->save($product);
/*Add Images To The Product*/
$imagePath = "sample.png"; // path of the image
$product->addImageToMediaGallery($imagePath, array('image', 'small_image', 'thumbnail'), false, false);
$product->save();