Mdkart

This plugin for WordPress automatically add the rel=”lightbox[ID-OF-THE-POST]” to images linked in a post 
ID-OF-THE-POST is unique per post so all images per post are grouped in one lightbox set. 
Works also with the gallery Fonction of WP 2.7 and in comments
Doesn’t add the the files required for Lightbox (it only add the rel lightbox tag)! You have to insert yourself in your theme the required files or use a plugin like Lightbox 2 WordPress Plugin or Lightbox 2.3 plugin for WordPress
<h3>Changelog / Todo</h3>
0.4.1 
* Optimizing the code 
0.4 
* Now Compatible with the Gallery Fonction of WP 2.7 
* Adds also rel tag in the comments 
0.3 
* Correction of a bug that added rel tag on links done on img tags. 
0.2 
* Correction of a small bug described by Kristof Mattei. Thanks to him for the help!
Thanks to all who sent bug reports and ideas for improvements. 
Please send me a mail if I forgot you to mention here.

Add Lightbox

This plugin for Wordpress automatically add the rel="lightbox[ID-OF-THE-POST]" to images linked in a post

Marco Milesi

WP Attachments enhances the download experience and file management in WordPress. It automatically displays attachments in your posts and pages, so you don’t need to manually insert download links. 
Easily attach, unattach, or reattach files directly from the media library.
Key Features: 
– 🤖 Automatically displays attachments after post content 
– ℹ️ Backend metabox for managing attachments 
– 🔃 Quick Attach, Unattach, and Reattach actions in the Media Library 
– 🔢 Download counter with anti-spam and logged-in user filtering 
– 🧑‍💻 Developer hooks and filters for customization 
– 🛍️ WooCommerce compatible 
– 🎨 Five icon packs to choose from 
– 📜 Supports posts, pages, and custom post types 
– 🎢 Customizable themes (title, date, size, caption, and more)
<a href="https://www.youtube.com/watch?v=J7gf0hxl_z8" rel="nofollow ugc">Video Overview</a>
<h3>Contributions</h3>
<ul>
<li>Part of <a href="http://www.wpgov.it" rel="nofollow ugc">WPGov.it</a>, providing open source solutions for Italian Public Government websites.</li>
<li>Thanks to the Italian community <a href="http://www.porteapertesulweb.it" rel="nofollow ugc">Porte Aperte sul Web</a> for beta testing and ideas.</li>
<li>Metabox based on <a href="https://wordpress.org/plugins/ij-post-attachments/" rel="ugc">IJ Post Attachments</a></li>
<li>Some icons by <a href="http://p.yusukekamiyamane.com/" rel="nofollow ugc">Yusuke Kamiyamane</a>.</li>
</ul>

WP Attachments

Nice and useful plugin. The only downside is that it lacks a setting to show image thumbnails instead of a generic image icon in the edit metabox.

Show thumbnails instead of icons in metabox

A powerful solution to manage and display your WordPress media attachments in posts and pages.

Add Lightbox vs WP Attachments