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?

After trying the other "fixes" and nothing worked, I went with what was previously suggested here. Located the Nat Geo folder (HD>Library>Screen Saver>Default Collections>1-National Geographic) and removed those pictures. This does require your Administrator Password. Then I moved the pictures I wanted into that folder. This works, but it is not a fix as the Screen Saver still defaults to that folder.

@Charley3's answer did not work for me, as I constantly update the photos I want in my screen saver. I took it one step further by setting up the mac to copy the photos from my folder to the default national geographic folder once a day on weekdays:

1. Open Terminal application
2. Type sudo crontab -e
3. Type your password

4. Type i and then paste the following: (03 7 is 7:03am, change to desired update time. 1-5 is Monday-Friday, change to desired days)

   03 7 * * 1-5 cp -a <path to screensaver photos desired>/* /Library/Screen\ Savers/Default\ Collections/1-National\ Geographic/

5. Type [esc] :wq and hit [Enter]

6. Type sudo crontab -l and verify the information pasted in is correct

Now I can have the photo folder that I change the contents of daily still be set as my default screensaver folder.

Edit: In addition, I selected the 'shuffle slide order' checkbox in screen saver settings but that too shifts back to the defaults. In order to check this box regularly, I used this question and added an additional line to the crontab:

03 7 * * 1-5 /usr/bin/defaults -currentHost write com.apple.ScreenSaverPhotoChooser 'ShufflesPhotos' -bool "true"

I had this same problem. For me it was caused by a configuration profile that was overriding my screensaver preferences. (I had installed the profile while going through Apples OS X Support Essentials book)

Here are Apples instructions :

Remove a configuration profile from your Mac

1. Choose Apple menu > System Preferences, then click Profiles. If you haven’t installed any configuration profiles, the Profiles pane of System Preferences isn’t available.
2. Select a profile in the Profiles list, then click Delete (-).