NASA on The Commons on Flickr
URL: https://www.flickr.com/photos/nasacommons
Copyright Info: https://www.nasa.gov/multimedia/guidelines/index.html
Flickr Terms: https://www.flickr.com/help/terms
NASA on The Commons is a great page on Flickr which gives people access to amazing photos of space. Many of the images on the page are public domain but those which are marked as CC.