If a screenshot from 'Fallout 4' helps to illustrate a story that CNN is trying to tell then they should use it. The purpose of illustrations is to help explain the story they are trying to tell. Just because an illustration is from a game does not automatically disqualify it from use by CNN.
Yes, CNN can use a video game as a source as long as it is made explicitly clear that the video game screenshot is for demonstration purposes only and that it is not a true graphical representation of events. Visual images are sometimes imperative to understanding and this use could be beneficial.
In today's day and age where the respectability of journalism is in question, it is not a good idea for CNN to use a game video as a source for a real news story. Video games by their nature are an alternative reality. It soils CNN's reputation by using this alternative reality to describe new story in actual reality.
Using a video game screenshot in an article about hacking is not a good idea. It gives readers a false impression of what hacking is and what it entails. It also makes hacking seem less real and disconnects the readers from it. For example, using a Call of Duty screenshot in an article about the Syrian war would make the war seem much less real and tangible, so for the same reason, CNN should not use a Fallout 4 screenshot when talking about hacking.