Where are currently set wallpapers stored in Mountain Lion?
Have a look at this hint: http://osxdaily.com/2011/11/20/show-the-location-path-of-current-wallpaper-in-mac-os-x/

You can (temporarily) show the location of the current desktop picture by executing the following command from the Terminal:

defaults write com.apple.dock desktop-picture-show-debug-text -bool TRUE;killall Dock


You can revert this behavior by executing the following:

defaults delete com.apple.dock desktop-picture-show-debug-text;killall Dock