Because the whole point was to make it easy for our families, viewers do nothing but visit a specific URL, and enter a password (chosen by the presenter). Presenters have a little more work (you need a google account, you need to install a greasemonkey script & you have to set a password for the slideshow).
As the slideshow controller, you run a greasemonkey script, that takes note of whatever picture you're currently viewing on flickr:
Slideshow viewers go to your personalized url, and sit back and watch, while, thanks to the magic of ajax, the page is updated every time you navigate to a new picture on flickr:
Works pretty great in combination with narration via phone/skype/google chat.