This Yahoo! Pipe produces an RSS feed of images in the most recent post on The Big Picture, Alan Taylor's selection of newswire images for the Boston Globe.
It fetches The Big Picture's RSS feed, selects the link for the first (most recent) item, then uses YQL to extract all the images from that post (select * from html where url="{item.link}" and xpath='//img[@class="bpImage"]'. It then creates an <img> element for each image, sets the src attribute to the URL of the extracted image, and adds it to the description field of the output.
The output is then available as an RSS feed, which can be added to the screensaver in OS X (System Preferences > Desktop & Screen Saver > Screen Saver > Pictures > MobileMe and RSS > Options… > + > Add RSS feed) or even as JSON for use in an embedded slideshow.
Update: In Snow Leopard's Screen Saver preferences, click the '+' icon below the list of screen savers and choose "Add RSS feed..." instead of the above.
View Source is available from the Pipe's page.