Lightbox is a cool feature that should be in every images in your website so when a visitor clicks an image, the website is darkened and only the image will be shown. In this way, readers can freely return to the post without any interruptions like the image opened in the new tab or is replaced with the post that they are reading. When you want to get lightbox to all your posts especially in your old ones, doing it manually is such a pain in the neck. So, here I have a simple code snipped that you just need to put in your functions.php file to add lightbox automatically to all your images.
Get the code snippet here: http://www.wprecipes.com/how-to-automatically-add-rellightbox-to-all-images-embedded-in-a-post