How to preserve wallpaper on secondary monitor after reboot This problem began when I upgraded to Yosemite.  I have two monitors connected to my Mac Pro.  Primary monitor uses default wallpaper, and secondary monitor uses a different wallpaper (solid gray).  Every time I reboot the system, the secondary monitor reverts to the default wallpaper, and Mission Control shows an additional "desktop" on the primary monitor, with a solid gray wallpaper.
After rebooting many times, the list of desktops on the primary monitor grows quite long if I don't delete them manually.

I have read a tip to fix this behavior by disabling this feature in Sys Prefs, but I don't want to do that.
Cross your fingers and stand on one leg.

More seriously, you probably need to delete related plists.  

Try deleting this:

/Users/&lt;your name&gt;/Library/Application Support/Dock/desktoppicture.db