I need to write a citation for an image -- this is not my paper, I'm just advising on the citation.
This image is the one that is being used.
Should I cite the book as the most important and like most stable source of the image, or just do the online image citation? Or, am I capturing the full provenance of an image: a retouched version of an image found online that came from this specific book?
I'm not so concerned with citation format (I'm using Medline), but making sure I think through the citation correctly for this scenario.
You need to cite the book, as that vouches for the provenance of the image (including that it is of who it is claimed to be).
If I'd done that retouching myself, I wouldn't mention it as it doesn't change the content (it's effectively undoing some of the effects of scanning an old image). A "(retouched)" might be appropriate, and certainly "(enhanced to show...)" if that was what you'd done.
If someone else had retouched images specifially for a paper I was writing, they'd deserve an acknowledgement, so here they deserve a mention. I would cite the original still, but with a note (or the equivalent in your referencing style). Something like "(via https://commons.wikimedia.org/wiki/File:Cajal-Restored.jpg, retouched by user Garrondo)" should be sufficient for academic propriety (and should cover the CC requirements were that an issue).