OK, here it is:
"You must own or control the copyright to all content you submit to Shutterstock. This means that you cannot submit work obtained from other sources (e.g., online image search results or websites), or incorporate such work into your content submissions, unless you have permission to do so."
Regarding NASA elements:
"If you submit images that incorporate NASA elements that are in the public domain, Shutterstock may accept the image as long as the following text is included in the title for that image: Elements of this image furnished by NASA.