function auto_set_featured_image_for_new($post_id) {
if (wp_is_post_revision($post_id) || has_post_thumbnail($post_id)) {
return;
}

$default_image_url = 'https://example.com/default.jpg'; // ← URL'yi buraya yaz

$upload_dir = wp_upload_dir();
$image_data = file_get_contents($default_image_url);
if ($image_data === false) return;

$filename = basename($default_image_url);
$file = $upload_dir['path'] . '/' . $filename;
file_put_contents($file, $image_data);

$wp_filetype = wp_check_filetype($filename, null);
$attachment = array(
'post_mime_type' => $wp_filetype['type'],
'post_title' => sanitize_file_name($filename),
'post_content' => '',
'post_status' => 'inherit'
);

$attach_id = wp_insert_attachment($attachment, $file, $post_id);
require_once(ABSPATH . 'wp-admin/includes/image.php');
$attach_data = wp_generate_attachment_metadata($attach_id, $file);
wp_update_attachment_metadata($attach_id, $attach_data);

set_post_thumbnail($post_id, $attach_id);
}
add_action('save_post', 'auto_set_featured_image_for_new');

Bunu temanızın fonksiyon dosyasına ekleyin,
eski yazılara eklenmesi için diğer yöntemle birlikte kullanın kaynak: https://rootali.net/wordpresste-one-cikan-gorseli-olmayan-yazilara-otomatik-gorsel-atama.slw