Forum Discussion
A little late to this party, so I rather doubt the OP is still looking for answers, but here's a couple good resources I've found for getting started:
Crash Course in Roku SDK - Not specific to Screensavers, but a great resource nonetheless.
Chris Taylor's Roku-GooglePhotos - This is both a slideshow app and screensaver. But I loved it because it really showed me how to create a settings menu for my screensaver and how to utilize the device registry to store the settings selections.
For what it's worth, I am hosting the photos for my screensaver on my own domain. I created a mySql database for the images so I can add categories and captions, and wrote a little xml api using php in order to serve the images to my screensaver.
Hope this helps folks! Good luck and happy coding!!