Change background image with cron, using images from a folder where the current background image is.

wallpaper_changer.sh

#!/bin/bash

PID=$(pgrep gnome-session | tail -n1)
export DBUS_SESSION_BUS_ADDRESS=$(grep -z DBUS_SESSION_BUS_ADDRESS /proc/$PID/environ | tr '\0' '\n' |cut -d= -f2-)


current_image=$(gsettings get org.gnome.desktop.background picture-uri | xargs echo)

if [ "$1" == "echo" ]; then
	echo $current_image
	exit 0
fi

folder=$(dirname "$current_image")
filename=$(basename "$current_image")	
last_image=$(ls -v $folder | tail -n 1)

if [ "$1" == prev ]; then
	first_image=$(ls -v $folder | head -n 1)
	if [ "$first_image" == "$filename" ]; then
		next_image=$folder/$(ls -v $folder | head -n 1)
	else
		next_image=$folder/$(ls -v $folder | grep -B 1 $filename | awk 'NR==1')
	fi
elif [ "$filename" == "$last_image" ]; then
	next_image=$folder/$(ls -v $folder | head -n 1)
else
	next_image=$folder/$(ls -v $folder | grep -A 1 $filename | awk 'NR==2')
fi

gsettings set org.gnome.desktop.background picture-uri $next_image

if [ "$1" == "trash" ]; then
	rm $current_image
fi