I was looking for a specific Acorn image, which I couldn't remember the filename of but I did remember the folder it was in as well as the text in it ("fx"). And then I also remembered that text boxes in Acorn files are indexed via Spotlight, and since I was already in Terminal I ran the following command in that folder:
$ mdfind -onlyin . fx firstname.lastname@example.org stylechain-layer.acorn email@example.com stylechain-layer-highlight.acorn …
And tada, there were the images I was looking for.
Doing this in Terminal is not necessary, I could have also easily done this via the Spotlight menu.