Photo stream images are not "on your computer" unless you selected to have them save to the event library. Can you see any of these images in "events" or "photos"? If not, then they is why you would not see them in the screen saver, or be able to add them to a slideshow.
This site contains user submitted content, comments and opinions and is for informational purposes only.
Apple disclaims any and all liability for the acts, omissions and conduct of any third parties in connection with or related to your use of the site.