Screen saver settings keep defaulting back to National Geographic Photos

I like to use my photo library for a screen saver (have 10,000 photos - it's nice to get to see them more often), but ever since I've downloaded Yosemite, every time the computer goes to sleep, it defaults back to the National Geographic photos (there are maybe 30 of them). Really, really frustrating, and I can't find where to change the default settings to try and make an adjustment. Anyone know how to change that default setting or have any other idea what to do?