Sorry for image heavy.
Well, there was a topic put in add-ons that supposedly changes desktop to the default BL screenies. Not only does he say you cannot use it on mac, but there is a way to do the EXACT SAME THING on mac without downloading any software for it.
Step 1:
Go to System Preferences (the light switch with the apple logo next to it).
Step 2:
Go to desktop and screensaver.
Step 3:
Click choose file. A menu should come up.
Step 4:
Choose Blockland folder, then choose screenshots but don't open it. BL will be located in the file you have it in (e.g. desktop, applications, etc., default is desktop).
This menu should come up:
Step 5:
Enable "change picture" and set to every five seconds.
EDIT: Did I mention it updates with every new screenshot you take?
