When you have no idea where the original wallpaper image is stored from your background, you can use the following command to display the full path to the currently active desktop image. This would be display directly on the wallpaper itself.
defaults write com.apple.dock desktop-picture-show-debug-text -bool TRUE
To make the command take effect, you need to restart the Dock.
killall Dock
To deactivate the path use:
defaults delete com.apple.dock desktop-picture-show-debug-text
Restart Dock
Thanks! This was a difficult trick to find. I had migrated a user and a bug in migration (most likely) somehow turned this on. I was googling around to find how the background image filename got overlaid on the desktop for a while before I found your post.