Desktop image is not showing in Mountain Lion with dual monitor

Solution 1:

Restarting the Dock seems to work for me, however I don't know what is causing this grey desktop background.

To restart the Dock, type this into Terminal and press enter:

killall Dock

Solution 2:

OS X allows you to select different images for each display. When you open the Desktop section in Desktop & Screen Saver Preferences, you should get one dialog on each screen for choosing a desktop image. Each dialog controls only the image for that screen, so make sure you're setting the image you want on both screens.

Solution 3:

I also experienced this problem and found this workaround:

• Open System Preferences -> Displays -> Arrangements
• switch on "Mirror Displays" and switch it off again

This is no solution for the problem of course, but at least the wallpaper is showing again without rebooting...