How to cite a retouched image that originally comes from an ebook on archive.org?
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).