Call this often to get a new wallpaper every day.

random-wallpaper.sh

#!/bin/bash
#
# random-wallpaper.sh
# Call this often enough to get a new wallpaper every day.

# Where the wallpapers are located
WALLPAPERS=~/pictures/wallpapers/r-wallpapers

# Where the last change date will be saved
REGISTRY=~/.random-wallpaper

# The date format
DATEFORMAT=%Y-%m-%d

# The actual command
if [[ "`cat $REGISTRY 2> /dev/null`" != "`date +$DATEFORMAT`" ]]; then
    wallpaper="`find $WALLPAPERS | grep -E '(\.png|\.jpe?g)' | shuf -n 1`"
    gsettings set org.gnome.desktop.background picture-uri "file://$wallpaper" && date +$DATEFORMAT > $REGISTRY
fi