I'm not sure if you got the information you were looking for, so I'll
post it anyway for the general public.  To load an image on your root
window add this line to the end of your .xsession file:

 xloadimage -onroot -fullscreen <gif_file_name> &

This is assuming of course you have the xloadimage client, and as
for the switches, I think they pretty much explain what is going on.
If you leave out the <&>, the terminal locks till you kill it.
(You already knew that though...)

Hope this helps.