desktop/screensaver prefs pane doesn't load

iMac (21.5-inch, Late 2009)

3.06 GHz Intel Core 2 Duo

4 GB 1067 MHz DDR3

Available: 310.87 GB

Capacity: 499.25 GB

Yosemite: 10.10.3

In System Prefs, clicking Desktop & Screen Saver I get a message saying "Could not load Desktop & Screen Saver preference pane." Other pref panes open.

Before that, I'd tried Control-click or Right-click "Set Desktop Picture." It didn't work from desktop so I placed 3 JPEGs in my Users/[myname]/Pictures folder. Didn't work. Now the pref is broken.