Download the plugin: http://leandrovieira.com/projects/jquery/lightbox/
Add the file paths to application.js and application.css. Note the order of how you enter them.
//= require jquery
//= require jquery_ujs
//= require lightbox/lightbox
*= require lightbox/lightbox
*= require_tree .
Be sure to do config.serve_static_assets = true in production.rb so that assets will get served out of public/assets in production.
Run rake assets:clean followed by rake assets:precompile