Noticed how 10.10 sets the login window to the last admin users background.
Want to change that so you select the background ? Easy!
The file is located in
/Library/Caches/com.apple.desktop.admin.png
The file HAS to be a PNG, and if you want to lock it so that it sticks for all users change the permissions to 555
chmod 555 /Library/Caches/com.apple.desktop.admin.png