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.
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