Open your theme’s function.php file and add the given function editing the thumbnail image URL (given in bold text) with your own image URL.
add_action( ‘save_post’, ‘wptuts_save_thumbnail’ );
function wptuts_save_thumbnail( $post_id ) {
$post_thumbnail = get_post_meta( $post_id, $key = ‘_thumbnail_id’, $single = true );
if ( !wp_is_post_revision( $post_id ) ) {
if ( empty( $post_thumbnail ) ) {
update_post_meta( $post_id, $meta_key = ‘_thumbnail_id’, $meta_value = ‘https://sangkrit.net/thumbnail.jpg’ );
}
}
}