NASA has put their entire collection of images, sounds, and videos of galaxies, planets and space missions online, publicly accessible and copyright free for anyone to view or use.images.nasa.gov
NASA has over 140,000 images, sounds, and video in their media library, now available for viewing or download, copyright free. You can choose to see images, videos and audio (or all three) when browsing the gallery, narrow a search by year, and sort by newest uploads or most popular.
In addition to the database of amazing space images there are other interesting items like missions, launches, events, astronauts, the International Space Station, SpaceX launches, and a few brief videos narrated by William Shatner that Star Trek fans might like.
Most search results have a useful pop up description so you can see at a glance what the item is all about. Images have EXIF data - if you like photography or are curious about the details of an image that's a good feature.
NASA also has two channels on Giphy full of animated GIFs and a Pinterest page:NASA GIFsNASA on Pinterest
All the content in the NASA media library is free with no copyright restrictions for non-commercial use:
“NASA content - images, audio, video, and computer files used in the rendition of 3-dimensional models, such as texture maps and polygon data in any format - generally are not copyrighted. You may use this material for educational or informational purposes, including photo collections, textbooks, public exhibits, computer graphical simulations and Internet Web pages. This general permission extends to personal Web pages.”
Commercial use has other guidelines, more details at NASA's Media Usage Guidelines