http://manifoldgarden.tumblr.com/
Make a folder like ~/Pictures/manifoldgarden and put check.sh in it. Run it to fetch some images with:
bash check.sh
Then go to System Preferences -> Desktop & Screensaver, add the folder to use as your wallpaper and set it to change every now and then.
open Terminal and run:
crontab -e
you'll want to add this line:
0 * * * * cd $HOME/Pictures/manifoldgarden && bash check.sh
If you don't know how to use the editor (vi) you can do this by pressing i, pasting in the line, pressing ESC and typing :wq