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.
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