Since I upgraded to Yosemite my screen saver keeps reverting to the default pictures

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.

Worked for me. Thanks Linc

I tried this several times, even deleted all the default screensavers, restarted the computer, emptied the trash, and still I cannot keep my iPhoto folder selection as a screen saver. I cannot upgrade iPhoto to "11". Right now it is running 9.6. And when I try to set the screen saver, the window still says 1-National Geographic, but there are no photos. This is quite frustrating.

Tried this and the fix only worked for an hour or two and then reverted back to National Geographic.

I have the same problem and am happy to try Linc's solution, but has Apple weighed in on this yet?

I gave up on Yosemite and the screen saver. I asked someone in my local Apple store and she said my processor (from 2010) is just not able to handle everything with Yosemite. For some reason every time I ask someone I get a different answer, but this one made sense for the first time. I downloaded a free "Screensaver" from the internet which only worked for a day or so. Then I downloaded the "Screensaver +" version - $2.99 from iTunes- and it has worked perfectly with my photo album for the past several weeks. I get the same photos I wanted, I only paid once, I have no more issues, and I am happy!

Simple solution: I discovered that if you simply leave the desk top photo of your choice on the desktop before shutting off your computer, that photo will stay. So, after you set your desk top screen to the photo of your choice, make sure that photo is dragged to the desktop and not left in a file.