If you are interested in programmatically retrieving photos for your website or application, check out Flickr's API. It's free! You are most likely going to be interested in the flickr.photos.search method.
If you are looking for the RSS functionality that was removed from the OS X screen saver system preference,
you can find the option hidden in iPhoto. In iPhoto, choose 'File' > 'Subscribe to Photo Feed...' and then paste the URL of the RSS feed in the box that pops up. Exit iPhoto after your computer thrashes for a few moments as the images are cached locally. The RSS feed will appear as a 'Photo Library' under 'Source' in the screen saver system preference. you're out of luck.