Automatically change OS X desktop background for all desktops + monitors?

Solution 1:

I have dual monitor Mac at work so I will be able to give it a try tomorrow (day off today).

I think if you were to compare the output of:

$ defaults read com.apple.desktop

Before and after having manually changed the wallpaper via:

System Preferences > Desktop & Screen Saver > Desktop

That, should, bring up a window in the second display allowing you to select required image. Once that is setup and working, compare the output from the original command and see if it gives you any clues. I shall do the same tommorow and play around it with it. See if I can't give you a more complete answer.