This thread has been changed because I felt it wasn't helpful enough and the original information was deleted.
To change the GDM image all you really have to do is right click on the desktop and hit "Change Desktop Background" and select the picture you want and then hit the "Make Default" button and both will change naturally. (The lock screen and GDM Login background)
Then just switch it back to your usual desktop wallpaper and go on about your day.