This lock dialog was orginally designed by alex57373203 and updated by etalyc.I have just simply created and changed the backgroud image to something which I personally prefer and want to share. The original theme was an NSA lock dialog, so that is why 'nsa' is used in file names and the gconf-editor.
1) Open up a terminal and type:
2) Then navigate to usr > share > gnome-screensaver
3) Copy the files 'lock-dialog-nsa.gtkrc', 'lock-dialog-nsa.ui' and 'nsa-images' to the gnome-screensaver folder.
4) Press Alt+F2 on your keyboard and type in 'gconf-editor'. Click on apps > gnomescreensaver and change default to nsa. To revert back to the default lock dialog, simply replace nsa with default.
5) If you do not have a user display picture then you need to go into 'nsa-images', delete or rename 'nsa_bg.png', and rename the image 'nsa_bg.png.noiconn' to 'nsa_bg.png' (without apostrophes obviously).
An easier alternative for doing step 4 is to run this command in the terminal:
gconftool-2 -s /apps/gnome-screensaver/lock_dialog_theme --type string "nsa"