MacOS – Desktop image is not showing in Mountain Lion with dual monitor

None of these other solutions work on Mavericks anymore because Apple moved the settings to a sqlite DB. But that's ok because now it's easier, the png can be anywhere in the filesystem, and all desktops (even virtual) are updated.

 #!/usr/bin/env bash
 sqlite3 ~/Library/Application\ Support/Dock/desktoppicture.db "update data set value = '/path/to/any/picture.png'";
 killall Dock;

Or, add it as a function to your ~/.bash_profile and call it as a terminal command with any non-relative path.

#   Update all Wallpapers
function wallpaper() {
    sqlite3 ~/Library/Application\ Support/Dock/desktoppicture.db "update data set value = '$1'" && killall Dock 
}

wallpaper ~/path/to/any/picture.png

Seems like if you set the first desktop to the picture you want and then go to each space and right click on the desktop and select "Change Desktop Background" it changes that desktop's background to the background of Desktop 1.

YMMV: This in 10.8.1, pretty sure it wasn't like that before (in 10.8).