Problem

CKEditor has a field where you can specify a custom class to use with an image. To use it you have to go to "advanced" tab and type the class into the "Stylesheet Classes" field. We want to have a default value in there to use in the Lightbox settings to open these images automatically with Lightbox.

Solution

http://drupal.org/node/314566
Scroll down to the section Modifying the source : Getting Lightbox to work automatically