Here's a world map showing current sunlight and cloud cover.
Here's a shell script to download the image:
curl -o ~/Pictures/earth/earth.jpg http://www.die.net/earth/rectangular_1600.jpg
Here's a tool (Cronnix) to add a shell script to the crontab so it runs every 15 minutes (use 0-59/15 for the minutes section, and * for the rest).
Then in System Preferences, set the Desktop picture to ~/Pictures/earth, set it to change every 5 mins, and you have a desktop picture of the Earth that updates every 15 minutes.