NASA

[Daddydavek]

The NASA website is an awesome resource and it's galleries have pictures many of which are free for use.  

 

Here is a link to their galleries:   https://www.nasa.gov/multimedia/imagegallery/index.html

 

I've found that the pictures help me visualize and therefore be able to write descriptions that are more believable and detailed than just what my imagination can provide and in addition have some real basis in reality.