I upgraded to 10.10.2 and I thought the problem had been fixed but it hasn't. This is what I have found out which sheds some light but I don't know how to fix it.
In the ~/Library/Preferences/ByHost Folder there is the screen saver files. The one in question I see is this one:
com.apple.ScreenSaverPhotoChooser.3894DB94-8127-583F-A292-E5634E257DF3.plist
After setting my screensaver information properly. the XML file contains this:
<plist version="1.0">
<dict>
<key>LastViewedPhotoPath</key>
<string></string>
<key>SelectedMediaGroup</key>
<string>2CE52ED9-1E40-4569-816A-1B5A446FC00F</string>
<key>SelectedSource</key>
<integer>1</integer>
<key>ShufflesPhotos</key>
<true/>
</dict>
</plist>
After the screensaver is run, it looks like this:
<plist version="1.0">
<dict>
<key>LastViewedPhotoPath</key>
<string>/path/to/last/image/(my proper image that it was viewing)</string>
<key>SelectedSource</key>
<integer>1</integer>
<key>ShufflesPhotos</key>
<true/>
</dict>
</plist>
After running it again, it turned to this.
<plist version="1.0">
<dict>
<key>LastViewedPhotoPath</key>
<string>/Library/Screen Savers/Default Collections/1-National Geographic/NatGeo03.jpg</string>
<key>SelectedFolderPath</key>
<string>/Library/Screen Savers/Default Collections/1-National Geographic</string>
<key>SelectedSource</key>
<integer>3</integer>
<key>ShufflesPhotos</key>
<true/>
</dict>
</plist>
Unfortunately, even if I copy the original plist back into the folder, the settings don't revert. But Obviously, I'm seeing the "SelectedMediaGroup" is missing after the screensaver runs once. and then it doesn't know what to do so it reverts to National Geographic as the default.