If you're using Drupal version 7 and are looking for Lightbox, it's not available. Lightbox 2 should work in Drupal 7, but it's currently, as of the writing, in Beta.
The best solution for those wonderful image galleries, with the main page shaded in the background, in Drupal 7 is the Colorbox Plug-in & Colorbox Module.
Follow these simple video install and configuration instructions, by the developer – Fredrik Jonsson and you'll be happy with the results.
Colorbox for Drupal 7, part 1: Installation and image field integration from Fredrik Jonsson on Vimeo.
Colorbox for Drupal 7, part 2: Inline images with the Insert module from Fredrik Jonsson on Vimeo.