At least once each day my screen saver/desktop picture reverts back to National Geographic and I go back to System Preferences and reset it.....
iMac
Try deleting the System Prefernces plist
See if the "fix" posted by big.josh will work for you:
Feb 17, 2015 5:48 AM
Re: screensaver reverts to default
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.
Why does my desktop/screen saver continually revert back to National Geographic?
