Q: screensaver reverts to default

Back up all data. Quit System Preferences if it's running.

Triple-click anywhere in the line below on this page to select it, then copy the text to the Clipboard by pressing the key combination command-C:

~/Library/Preferences/ByHost

In the Finder, select

          Go ▹ Go to Folder...

from the menu bar. Paste into the box that opens (command-V), then press return. A folder should open. If it does, look for a file with a long name that begins "com.apple.screensaver". There may be several such files. Move them all to the Trash.

Log out or restart the computer. Test. If the problem is resolved, empty the Trash and recreate your settings in the Screen Saver tab.

I have been experiencing the same problem for awhile now.  Actually, to the best of my memory, I believe it might have even started before updating to Yosemite.  Before, I read Linc's response, I tried another fix.  Because I had at times in the past noticed that having iPhoto open would sometimes seem to be the culprit that caused my screensaver to revert back to a default option, I decided to set-up a folder on my desktop called "Christmas Slide Show".  I then dragged all of the pix in my existing iPhoto "Christmas slide show" album into the aforementioned desktop folder.  This removed iPhoto from the equation.  Then, of course, you would select the desktop folder in Systems Preferences as your screensaver.  I experienced no problems after that.  It worked perfect for a couple of days.  But then, after I read Linc's fix, it seemed apparent that my fix was only a work around... where his was an actual solution that enabled iPhoto & Systems Preferences to perform their intended functions.  So, i tried it.  Thus far, Linc's solution is working great for me.  I only responded because it did not seem to work for everyone.  For those that are still having troubles, you might give my work-around a shot... at least till Apple comes up with a permanent solution that works for all.

2.4GHz Intl 2 Duo, 24", 6GB, 2TB, Mac OS X (10.10.1)

"Triple-click anywhere in the line below on this page to select it, then copy the text to the Clipboard by pressing the key combination command-C:"

what does this mean?

I tried Linc's solution, it worked for a couple of hours. I'm now back to National Geographic slideshow. Any other suggestions out there?

I found an idea in another discussion that has worked for over a week.

Just leave System Preferences open after you've set it up the way you want, then it'll stay on the album you selected.  It even seems to stay on it after a few days if you close it up then.  I haven't messed around enough to know the exact conditions but it's been showing the pictures from my album for a week or more now and the only thing I did different was to leave it open.

I half tried the above solution -- deleting those files with System Preferences closed and then setting up the screen saver again, but I didn't want to either reboot or login and off again right then and it didn't work.

I had the same issue and tried this fix. I wasn't convinced it worked for me.

However, I also found that when selecting the source of photos for the screen saver it makes a difference which folder I select...

If I select the 'iPhoto' library icon that first comes up (in the hope of seeing all my Photos) it reverts to National Geographic when the screensaver runs.

But if I click the triangle to the left of the 'iPhoto' icon, and then select the 'Photos' icon from the drop-down list that appears, the screensaver does not revert to displaying National Geographic photos.

I don't see any triangle near the iPhoto drop down??