This is a simple script that, when run, sets the background to a random picture in a directory. You can specify the directory and the filetype of the pictures by editing the variables at the top of the script. Put this code in a file named randbg.sh.

#!/bin/bash

IMAGE_DIRECTORY=/home/michael/img/wallpaper/current
FORMAT='*.jpg'

NUM_OF_IMAGES=`ls $IMAGE_DIRECTORY/$FORMAT | wc -l`
if [ $((NUM_OF_IMAGES)) -gt 0 ]
then
   ((N=RANDOM%NUM_OF_IMAGES+1))
   IMAGES=`find $IMAGE_DIRECTORY -name "$FORMAT" -printf "%p\r"`
   IMAGE=`echo $IMAGES | awk -F"\r" '{ print $(n) }' n=$N`

   echo "Setting background to '$IMAGE'"
   fbsetbg "$IMAGE"
fi