Someone

over all windows--> xwinwrap -ni -argb -fs -s -st -sp -a -nf -- /usr/lib/misc/xscreensaver/glmatrix -root -window-id WID
on desktop--> xwinwrap -ni -argb -fs -s -st -sp -b -nf -- /usr/lib/misc/xscreensaver/atlantis -window-id WID

xwinwrap -ni -o 0.6 -fs -s -st -sp -b -nf -- mplayer -wid WID -quiet movie.mpg 

Slide show fading in and out on desktop. (from http://ubuntuforums.org/showpost.php?p=881849&postcount=1318 )
xwinwrap -ni -fs -s -st -sp -b -nf -- /usr/lib/xscreensaver/glslideshow -window-id WID -duration 30 -pan 15 -fade 10 -clip -delay 50000