Add wp-image-ID class to images in content without them, as long as they are a WP upload. (UNTESTED!)

add_wp_image_id_class.php

<?php

function wp_add_image_id_to_img_tags( $content ) {
	if ( ! preg_match_all( '/<img [^>]+>/', $content, $matches ) ) {
		return $content;
	}

	$selected_images = array();
	/*array*/$uploads = wp_upload_dir();
	/*fullpath*/$uploads = $uploads['basedir'];
	/*relpath*/$uploads = trim(str_replace(ABSPATH, '/', $uploads), '/');
	/*regexp*/$uploads = str_replace('/', '\\/', $uploads);
	// $content = $content . '<!-- REGEXP: /'. $uploads .'/i -->';
	foreach( $matches[0] as $image ) {
		if ( false === strpos( $image, ' srcset=' ) && !preg_match( '/wp-image-([0-9]+)/i', $image, $class_id ) && preg_match( '/src=["\'][^"\']*'. $uploads .'\/([^"\']+)["\']/i', $image, $src ) ) {
			$selected_images[ $image ] = $src[1];
		}
	}

	$index = array();
	if ( count( $selected_images ) > 1 ) {
		global $wpdb;
		$srcs = "'". implode("', '", array_map('esc_sql', $selected_images)) ."'";
		$attachment_list = $wpdb->get_results("SELECT post_id as id, meta_value as src FROM {$wpdb->postmeta} WHERE meta_value IN ($srcs) AND meta_key LIKE '%_attached_file'");
		foreach ($attachment_list as $item) {
			$index[ $item->src ] = $item->id;
		}

		foreach ($selected_images as $image => $src) {
			$selected_images[ $image ] = array_key_exists($src, $index) ? my_add_class($image, 'wp-image-added wp-image-'. $index[ $src ]) : false;
		}

		$selected_images = array_filter($selected_images);
		$content = strtr($content, $selected_images);
	}

	return $content;
}

function my_add_class($htmlpart, $classes) {
	if ( false !== strpos($htmlpart, 'class=') ) {
		$htmlpart = preg_replace('/class=(.)/', 'class=$1'. $classes .' ', $htmlpart);
	}
	else {
		$htmlpart = explode(' ', $htmlpart);
		$tag = array_shift($htmlpart);
		array_unshift($htmlpart, 'class="'. $classes .'"');
		array_unshift($htmlpart, $tag);
		$htmlpart = implode(' ', $htmlpart);
	}
	return $htmlpart;
}

add_filter('the_content', 'wp_add_image_id_to_img_tags', 9);