What is research misconduct of Xuetao Cao about? Recently on social media, news broke that Xuetao Cao duplicated or manipulated images in more than 50 papers of his. These accusations were brought to the public by Elisabeth Bik and explained in great details in following article

using social media and pubpeer website they notice duplication in images.

as someone who finished MSc in Immunology I can't understand what is wrong with images, they look credible to me, although there might be some software that can tell difference. How is possible that so many figures got duplicated and no-one from peer reviewers noticed? What is this scientific misconduct about? If it is really one after all.
Without considering the actual case. A referee is not an investigator/detective.  This is why figures/tables/photographs might be composed or manipulated without being noticed at first. 

It is right what you say: they look credible and likely make sense in the context of the work. Though, some entries look copied and pasted. This is a recurrent case. Sometimes the authors take a pace in which even ex-novo fabrication of data is time and energy consuming.

An example of how things can go is this: https://en.m.wikipedia.org/wiki/Schön_scandal