I looked at what effect setting the permissions of the parent folder where all the .png images are stored had on the application. You should have noticed that your desktop no longer updated with a refreshed image as the application can no longer write the new image to that directory and subsequently refresh the image displayed on the desktop with the new one.
To the developer, the way in which this should be implemented is that older images are not stored separately, but either overwritten with the new image, or prior images be erased. When you're on a 2560 x 1600 display and refreshing every 30 min, it takes only a short period of time before this directory becomes a depository for a large amount of wasted hard disk space.
(Version 2.0b02 Universal)