I've seen many stuff here about it but all I want to do is delete this big 2 MB image which loads everytime my Ubuntu boots. As the general law says these things slows down the system. On boot it flashes and then get changed to my current desktop wallpaper. I don't want it anymore. May be I want to add this light weight image instead of it. Or can't I just get rid of it completely and only my desktop wallpaper is used in the login screen background?
This file's full path is /usr/share/backgrounds/warty-final-ubuntu.png. It is part of ubuntu-wallpapers package. I don't recommend uninstalling that package because you'll end up with unmet dependencies problems, but you could just remove the file:
or replace it with your black background:
Keep in mind that if ubuntu-wallpapers package gets updated the original file will be restored, then you'll have replace (or remove) it again.