All writing, imagery, etc. produced by the federal government is automatically in the public domain, available for anyone to use, share, remix, etc. For example, here’s a NASA page with some 1970s concept images of space colonies: https://settlement.arc.nasa.gov/70sArtHiRes/70sArt/art.html (via @AlanRalph).
I’m both a huge fan of poster art and a huge geek about travel to fantasy locations, so I’m doubly entertained by this batch of posters from the fun-loving artists at NASA advertising a bunch of possible travel locations in our solar system and beyond, available in resolutions high enough for printing at poster size:
Source: Visions of the Future
Image courtesy NASA/JPL-Caltech.