The Astronomy Picture Of the Day Web site literally digs into the vast archives of astronomy-related imagery and selects one photo to present every day. In addition to providing awesome desktop backgrounds, there is a small paragraph with each photo explaining exactly what you are seeing and providing links which let you dig deeper into the concepts behind the photo.
While I don’t select every day’s photo to add to my background collection, I do take the time to read the descriptions of every photo presented. I keep up to date by using their RSS feed.