This has been asked for several times and all the threads have been closed. I would like to renew the idea: wordpress.com needs a gallery like Lightbox. There’s a great demand for a better way to present photographs.
For fear of hearing “it’s been asked!”, I have to second this motion :) Something as simple and small as a jquery-style picture enlargement would work well. Just anything to not leave the current page to view an image would work, IMHO.