I was wondering the same thing about Lightbox… image display is the one thing that is preventing me from creating an artist portfolio site on WordPress.com.
Can WordPress.com engineer your own version of Lightbox? The current image display options are so limited; click-to-enlarge takes you to a separate page, from which you have to use the back arrow to return to original page :(.
I know Lightbox is available on wordpress.org, but I don’t think I’m smart enough to use that one yet. I would happily pay extra for such an image display option on WP.com.