I often surf by NASA's website for the Image of the Day, but NASA's site holds an inexhaustible supply of imagery. One of my favorite sections is their historical collection, Great Images In NASA:
NASA is a taxpayer-supported agency, so these images appropriately have been placed in the public domain. They can be used for most purposes (that's you, bloggers!) without prior permission as long as they aren't copied in a manner implying NASA's endorsement of a particular product.
If you're in need of space & aeronautics images, GRIN should be one of your first stops. You'll find the same photos spread elsewhere across the web, as happens to public domain, but GRIN presents them with the original detailed captions and in astoundingly high resolution.