This one is Mac OS X only. (Funny how I know a lot more astronomers with Macs than any other group of people). I was tinkering about earlier and created this automator script, which changes my desktop image to be today’s APOD (Astronomy Picture of the Day).
To do this yourself, or to expand upon and modify this script, download the workflow file here.
To set this up to run everyday, open it up in Automator. Click ‘Run’ to test that the script works (warning: it could take a minute or two to process, but this ok). You might also want to to change the folder into which the picture files will be saved. Then chose File –