NASA makes their entire media library publicly accessible and copyright free.
This is a good article from DIY Photography.
No matter if you enjoy taking or just watching images of space, NASA has a treat for you. They have made their entire collection of images, sounds, and video available and publicly searchable online. It’s 140,000 photos and other resources available for you to see, or even download and use it any way you like.
Here’s a link to the NASA Image and Video Library