The plugin that you're using, how does it target the images for the lightbox - does it use a class or rel target or something? As long as we can see how it's targeting images I can show you where to go into the theme code and modify the featured image to work with the plugin
where can i check this?
I'm using the plugin simple lightbox http://archetyped.com/tools/simple-lightbox/ Maybe You are familiar with this plugin
I'm not sure, you'd need to check out the plugin documentation and/or contact the plugin dev support...