Adds a shortcode that inserts responive images

WPMUDEV_Responsive_Images.php

<?php
/**
 * Plugin Name: WPMUDEV Responsive Images
 * Version:     1.0
 * Description: Adds a shortcode that inserts responive images
 * Author:      Panos Lyrakis (WPMU DEV)
 * Author URI:  https://premium.wpmudev.org/profile/panoskatws
 * Plugin URI:  https://premium.wpmudev.org/
 * License:     GPL2
 * License URI: https://www.gnu.org/licenses/gpl-2.0.html
 */

if( ! class_exists( 'WPMUDEV_Responsive_Images ') ){

	class WPMUDEV_Responsive_Images{

		private static $_instance = null;
		private static $_default_size;

		public static function get_instance() {
            if ( is_null( self::$_instance ) ) {
                self::$_instance = new WPMUDEV_Responsive_Images();
            }

            return self::$_instance;
        }


        private function __construct() {

        	self::$_default_size = apply_filters( 'WPMUDEV_Responsive_Images/default_size', 'thumbnail' );

        	add_shortcode( 'WPMUDEV_Responsive_Image', array( $this, 'show_image_sh' ) );
        }

        public static function show_images( $attachment_ids, $size = '', $return = false ){

        	$images = '';

        	if( is_numeric( $attachment_ids ) ){
        		$attachment_ids = array( $attachment_ids );
        	}

        	foreach( $attachment_ids as $attachment_id ){
        		
        		$images .= wp_get_attachment_image( $attachment_id, $size );

        	}

        	if( $return ){
        		return $images;
        	}
        	echo $images;
        }

        public function show_image_sh( $atts ){

        	if( ! isset( $atts['images'] ) ){
        		return;
        	}

        	$atts = shortcode_atts(
			array(
				'images' => '',
				'size' => self::$_default_size,
			), $atts, 'WPMUDEV_Responsive_Image' );

        	$attachment_ids = explode( ',', $atts['images'] );

        	return self::show_images( $attachment_ids, $atts['size'] );

        }

	}

	add_action( 'plugins_loaded', function(){
		$GLOBALS['WPMUDEV_Responsive_Images'] = WPMUDEV_Responsive_Images::get_instance();
	} );

}

You can use shortcode with image id:
[WPMUDEV_Responsive_Image images="876"]
Or several image ids:
[WPMUDEV_Responsive_Image images="876,844,824"]
and also add size attribute:
[WPMUDEV_Responsive_Image images="876,844,824" size="full"]