Comment 7 for bug 1147914

Please see the configuration details on this post: http://smdavis.us/2013/12/28/lightdm-gtk-greeter-1-7-0-released

Basically, in lightdm-gtk-greeter.conf, you set the position setting:

position=x y

Where x and y are the x/y coordinates, and can be relative positioning (positive numbers are indexed from top/left, negative botton/right) or percentages.