Mihai Parparita
wrote a bookmarklet that loads the feed for the current site in
Google Reader Play. Mihai suggests to use the bookmarklet for web pages that have a lot of photos, like
Flickr profiles or
comic feeds.

Google Reader Play creates a slideshow from any feed or collection of feeds, but I don't think it's useful as a standalone app. Google Reader could detect photo feeds and integrate Reader Play as a new view.