Disclaimer: I am not a lawyer.
"Fair use" is a tricky thing, and it's not clear where the boundaries of it are. However, fair use really only applies when determining whether or not something is copyrighted. It doesn't mean you can use something without attribution. As an answer by Gilles on Meta Stack Exchange says,
Plagiarism is a whole different matter from copyright. If someone posts an image without mentioning who the author of the image is, this is plagiarism. (It is also usually a copyright violation, but again this is not a point to pursue as a third party.) Plagiarism is forbidden on Stack Exchange.
Even if you did attribute the image, this still might not be fair use. An answer by JoErNano states
Therefore, according to my interpretation, using that image [referring to his use of an image in an answer of his] was indeed a copyright breach, regardless of it being referenced and linked to. Indeed, I was neither commenting, nor criticising, nor making a parody of the copied content. I was using that image without permission to enhance the content of my answer.
That case seems pretty similar to yours.
In general, always, always, always attribute your images. A lot of people don't do this; I'm likely missed it on some of my answers, in the past. At the same time, though, just because you see people jaywalking doesn't mean you should do it yourself. You could still get hit by a
car DMCA takedown. Here's an example of how I like to attribute images:
[![Comparison of conduction velocity in myelinated and unmyelinated neurons](https://upload.wikimedia.org/wikipedia/commons/4/4a/Conduction_velocity_and_myelination.png)](https://commons.wikimedia.org/wiki/File:Conduction_velocity_and_myelination.png)
<sup>Image courtesy of Wikipedia user WillowW under the [the Creative Commons Attribution 3.0 Unported license](https://creativecommons.org/licenses/by/3.0/deed.en).</sup>
Image courtesy of Wikipedia user WillowW under the the Creative Commons Attribution 3.0 Unported license.
The image links to the license in question (in this case, Creative Commons Attribution 3.0 Unported (CC BY 3.0) ) and gives the name of the creator.
In this case, it's worth noting that NASA's image use policy is generally very lenient:
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.
. . .
NASA content used in a factual manner that does not imply endorsement may be used without needing explicit permission. NASA should be acknowledged as the source of the material.
Essentially, you should acknowledge that the creator of the image is NASA, but you don't need to link to a license, as the work is probably in the public domain (but you should always check!) - something you may want to note in the answer.