It took a while for me to find out how to change the login window background image since it has changed so much since OS X Mavericks. I thought I would share how to do this from a terminal window since I found it incredibly useful.
com.apple.desktop.admin.png is the default Yosemite login window background image. When you delete this file and restart the machine it will be recreated with the image located at /System/Library/CoreServices/DefaultDesktop.jpg.
Since we linked /Library/Desktop\ Pictures/YourNewImage.jpg to /System/Library/CoreServices/DefaultDesktop.jpg, the new background image will be whatever image you used as YourNewimage.jpg.
# Loginwindow background settings # sudo ln -sF /Library/Desktop\ Pictures/YourNewImage.jpg /System/Library/CoreServices/DefaultDesktop.jpg sudo rm -rf /Library/Caches/com.apple.desktop.admin.png