If you want to add watermark to the specific product image programmatically, then use below script.

<?php 
/*
* @ custom script to add watermark image on the product image that has -f in the product image name
*/
use MagentoFrameworkAppBootstrap;

require __DIR__ . '/../app/bootstrap.php';
 
$bootstrap = Bootstrap::create(BP, $_SERVER);
$objectManager = $bootstrap->getObjectManager();
 
$state = $objectManager->get('MagentoFrameworkAppState');
$state->setAreaCode('frontend');
 
try
{   
    $objectManager = MagentoFrameworkAppObjectManager::getInstance(); 
    $categoryFactory = $objectManager->get('MagentoCatalogModelCategoryFactory');
    $category = $categoryFactory->create()->load(4); //Parent category
    
    /*
    * @ $cateid array of category IDs
    */

    $cateid = $category->getAllChildren(true); // get all subcategory id

    $cateinstance = $objectManager->create('MagentoCatalogModelResourceModelProductCollectionFactory');
    $allcategoryproduct = $cateinstance->create()->addAttributeToSelect('*')->addCategoriesFilter(['in' => $cateid]);

    $fileSystem = $objectManager->create('MagentoFrameworkFilesystem');
    $mediaPath = $fileSystem->getDirectoryRead(MagentoFrameworkAppFilesystemDirectoryList::MEDIA)->getAbsolutePath();    

    $productimages = array();
    foreach ($allcategoryproduct as $products) {

        $product = $objectManager->create('MagentoCatalogModelProduct')->load($products->getEntityId());

        $productimages = $product->getMediaGalleryImages();
        

        if(!empty($productimages)){             
            foreach($productimages as $productimage)
            {   
                $imgname = substr(strrchr(rtrim($productimage['file'], '/'), '/'), 1); // Get specific image name
                if(strpos($imgname, '-f') !== false){
                    $fullpath = $mediaPath.'catalog/product'.$productimage['file'];
                    if(file_exists($fullpath)){
                        watermark_image($fullpath, dirname(__FILE__)'/watermark-f.png', $fullpath);
                        echo $productimage['file']."n";
                    }
                
                }
                
            }
        }
    }    
 
}
catch (Exception $e)
{
    echo $e->getMessage();
    exit;
}

/*
* @ watermark_image function for merge watermark image to the product image
*/
function watermark_image($target, $wtrmrk_file, $newcopy) {
    $watermark = imagecreatefrompng($wtrmrk_file);
    imagealphablending($watermark, false);
    imagesavealpha($watermark, true);
    $img = imagecreatefromjpeg($target);
    $img_w = imagesx($img);
    $img_h = imagesy($img);
    $wtrmrk_w = imagesx($watermark);
    $wtrmrk_h = imagesy($watermark);
    
    imagecopy($img, $watermark, $img_w - $wtrmrk_w, $img_h - $wtrmrk_h, 0, 0, $wtrmrk_w, $wtrmrk_h);

    imagejpeg($img, $newcopy, 100);
    imagedestroy($img);
    imagedestroy($watermark);
}

?>