By their nature, few images can compete with images gleaned by NASA of our planet, our solar system and beyond. Their Facebook page is a good place to start exploring, however a visit to the official NASA site allows you to go deep into the images.
I’ve been fortunate when working with NASA astronauts to have had access to their personal image collections from the International Space Station and brilliant images they have been.
NASA website: www.nasa.gov
NASA Facebook page: www.facebook.com/NASA
NASA on SoundCloud: NASA stream on SoundCloud – Hear the world’s sounds
NASA CC Images on Flickr: NASA on The Commons