As an editor, what measures can I implement to prevent image/photo manipulation?
I think a search term for this type of manipulation detection is image forensics.  Matthias Kirchner: Notes on Digital Image Forensics and
Counter-Forensics may be a starting point. 

What can you do?

I think a first step is to communicate clearly what image manipulations* are acceptable and which are not:


is it acceptable to adjust brightness and contrast?
correct unequal illumination?
whitelight correction based not on a whitelight measurement but on parts of the image?
digital sharpening or other enhancement filters?
where's the border between an inset picture and a fraudulent manipulation?


and so on.

Who should detect this?

I agree that the reviewer should have commented and asked about the pictures above, however my experience as reviewer is that I often get pictures in ridiculously low resolution (I comment on that). I'm not sure, but I think that the publishers may ask for high resolution pictures when they spot such problems in the production process. However, that would mean that the reviewers may not have seen the actual picture that is used for printing. 



* manipulation in the data analysis sense: calculations that change the information content (e.g. enhance contrast), and cannot be easily undone (as opposed to a transformation where the back transformation is easily possible, e.g. rotation), not in the sense of fraud.