Themes are located in /usr/share/lxdm/themes
lxdm general configuration file is /etc/lxdm/lxdm.conf
and the default theme (Sabayon) is looking for its background in /usr/share/backgrounds/
, may be you could do some little script to get a random bg each time you boot.
I did some simple modification too in mine: http://forum.sabayon.org/viewtopic.php?p=140863#p140863
but i didn't go deeper. In the lxde official home
there is few documentation.