I've upgraded to Yosemite. Now my Screensaver is goofy! Why does my screensaver continually revert to the default when I've changed it several times, to use My Photos? This is very frustrating!
Try to delete the preference files:

/Users/[home folder]/Library/Preferences/ByHost/com.apple.screensaver*.plist


Open a Finder window, and then open the Go menu. When displayed press the &#x2325; key and appears an option called Library. Go to this option and your finder must be on Library folder. Then open Preferences, then ByHost and delete files starting with com.apple.screensaver and extension plist.

Then restart.