First select one photo of your image to put in logon screen, it should be dimension 800*600 pixels size 70.2 (appromimatley) and save in JPEG imag mode and give your photo name as backgrounddefault and put it in pictures place

What is Ubuntu Tweak :- Ubuntu Tweak is an application designed to config Ubuntu easier for everyone. It provides many useful desktop and system options that the default desktop environment doesn't provide. At present, it's only designed for the Ubuntu GNOME Desktop, and always follows the newest Ubuntu distribution.
LICENSE
Ubuntu Tweak is a free software, which means you can redistribute it and/or modify it under the terms of the GNU General Public License published by the Free Software Foundation.
Features of Ubuntu Tweak
View of Basic System Information(Distribution, Kernel, CPU, Memory, etc.)
GNOME Session Control
Auto Start Program Control
Quick install popular applications
A lot of third-party sources to keep applications up-to-date
Clean unneeded packages or cache to free disk space
Show/Hide and Change Splash screen
Show/Hide desktop icons or Mounted Volumes
Show/Hide/Rename Computer, Home, Trash or Network icon
Tweak Metacity Window Manager's style and behaviour
Compiz Fusion settings, Screen Edge Settings, Window Effect Settings, Menu Effect Settings
Set the Shortcuts to archive quicker access your favourite applications
GNOME Panel Settings
Nautilus Settings
Advanced Power Management Settings
System Security Settings
And many other in process...
you want to Download Ubuntu Tweak latest version go to this webpage http://ubuntu-tweak.com/

after installing Ubuntu Tweak in your system open that and you will fiend five options that is Overview, Apps ,Tweaks, Admins, & Janitor.

open 3rd option Tweaks, in that go to Login Setting and open it

now you will see option in disable mode first you have to enable it, you go to Unlock button on right hand side top of the box and unlock it.

if you press unlock button you will get Authenticate box in that you have to give your password and press authenticate button

now you will see all options will be enable mode

now you will see Guest account by default it is in on mode. if you want to desable it just change to off mode.

in that same way Draw grid put it in off mode

just click on photo box area under the “click the button on change the login screen background”

now you will see a box choose a new background image, now you have to go home folder and pictures and select you photo now and press OPEN button and close all and restart your computer

now you will see your photo on loging screen

How to set your own image at windows 7 logon screen

First select one photo of your image to put in logon screen, It should be dimension 800*600 pixels size 70.2kb (appromimatley)
save as JPEG Image mode. Put your photo name has “backgrounddefault”.

Now goto C drives and double click --> goto windows folder --> sys32 folder --> oobe folder and open it.
You will see info name folder [in case it is not their just create a folder info
And open it.
After opening info folder you have to Create a new folder and change the folder name as backgrounds and past your photo on this folder close it.

Next got to RUN and type regedit and press yes button
Now goto --> HKEY_ LOCAL _ MACHINE --> soft ware --> Microsoft --> windows --> current version --> authentication --> logon UI –-> background [ on right side you will see OEMBackground.]

Now double click on OEMBackground it will open Edit DWURD (32 – bit) value box
In value date by deflate it is 0 now you change to 1 and press ok button and close
Now restart your system and enjoy.