LightDM wallpaper is worse now after the last update/upgrade

[UH]

2012/1/20 amjjawad HOOHAA <amjjawad at gmail.com>:
> Hello Everyone,
>
> sudo apt-get update
> sudo apt-get upgrade
> sudo apt-get dist-upgrade
>
> After running the above commands on Lubuntu 12.04 (No, I'm not testing the
> daily iso, this installing was done 2 weeks or more), I logged out to check
> the wallpaper and it's ALL black now with light gray bar at the top. I can't
> see anything but these two colors + the mouse cursor. That is all.
>
> [. . . ]

Btw, how do you change the login wallpaper in lxdm (i'd like to use
the "Natty_Bug.png")? I'm familiar with lightdm (which i use on Mac
running LXDE) but not lxdm, which i remember quite complicated when i
tried to try it out instead of lightdm. TIA