Edit as necessary, requires hsetroot, mplayer, and a working xvidix on your graphics card (for video).

# Set Wallpaper

# If $1 is not empty
if [ -n "$1" ] ; then
	case "$1" in
	  	# Grab last set wallpaper from file and store in $WALL
	   	WALL=`cat ~/desktop/lastwall`
	   	# Set $WALL from command line and write to file for future use
		echo $1 > ~/desktop/lastwall

	case "$WALL" in
			# $WALL is an image file, so use hsetroot to set it as the background
			echo Setting with hsetroot...
			hsetroot -full "$WALL"
			# $WALL is likely not an image, so attempt to play with mplayer
			echo Setting with mplayer...
			# Blank out background first
			hsetroot -solid "#000000"
			# "Play" background using colorkey
			sudo mplayer -fs -vo xvidix -rootwin -colorkey 000000 -quiet "$WALL" &

	# Return good
	exit 0
	echo Please specify an image or a video file to set, or use "last"

	# Return bad
	exit 1

To use, run setwallpaper.sh /path/to/image/or/video/file or setwallpaper.sh last