I had remembered seeing a photo in a thread but I couldn't remember which thread it was in. Since images are always external (e.g., on imgur, or elsewhere) I didn't think there was any way to Google search for them. But, I was wrong.
Simply adding: site:bitcointalk.org when searching Google Images gives a Pinterest-like view of the forum:
Even though the images are links to external sources, Google indexes them just as if they were hosted on BitcoinTalk.org. Fantastic!
Then the normal search filtering is possible, so I can click the "Past week" filter to see just recent images.
Then when I click to view the image I was looking for, in the top right is the "Website for this image" link, which brings me to the exact post in the thread with the link to the image.
This is a great new way (new for me, at least) to navigate the forum!