Location of Account/ Login picture on Mac OS X 10.12 Sierra
There is a terminal command that will print the path of the desktop picture on the picture itself. Using the terminal enter the following command:

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


Your desktop picture should now show the directory path to its location. When you get the location of the image you can turn this off by typing:

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