If you want to randomly change the wallpaper on mac osx you can use the built-in functionality. However, if you want to pick an image from a subdirectory you need to use this little script.
Download the script desktop_changer.py and save it to ~/bin/.
I followed the instructions on this page: http://www.davidlanier.com/story/cron-and-launchctl-on-mac-os-x-105-leopard.
Put local.desktop_changer.plist in ~/Library/LaunchAgents. Note, you'll have to edit line 12 and put the correct path in.
Make the following file: /Users/<your username>/.launchd.conf
load /Users/<your username>/Libary/LaunchAgents/local.desktop_changer.plist
Make sure everything is working by:
If you get an error about appscript you need need to install it via:
sudo easy_install appscript
Note, you'll probably have to lock down file permissions via:
cd chmod 644 Library/LaunchAgents/local.desktop_changer.plist chmod 644 .launchd.conf