Lee's answer is great (I'm glad to learn about the `photoID` URL hack). Here's how I found the answer:

1. Perform a Google "Search by Image".

2. The search results led me to a couple articles using the same image. Hopefully, I'm looking for a page or article that credits the photographer and links directly to the hosted image (Flickr, in this case).
    * At the time of this writing, the best result (at the time of this writing) leads to a [JSTOR article](https://daily.jstor.org/amoebas-are-smarter-than-they-appear/) that credits "Flickr", and links directly to Norbert Hülsmann's Flickr page of the image. Done.

3. However, sometimes the answer doesn't come so quickly. When I first searched for this image a couple days ago, that wasn't the first result. Instead, one of the first results was an [article at Quartz.com](https://qz.com/301402/what-yellow-slime-yes-slime-can-teach-your-organization/), which had the image and credited "Norbert Hülsmann/Flickr", but didn't link to the Flickr image.

4. After finding the photographer, adding their name to the Google Image Search produced a link to a [blog article](https://champagnewhisky.com/tag/physarum-polycephalum/) that credited Hülsmann, and also linked to the Flickr page. Done. (again).

In this example, the path was unnecessarily circuitous (for explanation purposes), but this is path sometimes required to figure out copyright and/or licensing for images. Search, refine, repeat.