A good example of this is television news. The medium of television and its demands shape the sorts of messages that are carried on it. This can happen in at least two ways.
First, news that is covered on television needs to be something that has pictures (and preferably dramatic pictures) to go with it. Because television is a visual medium, TV news needs to have pictures to keep viewers interested. This means that the sorts of stories that tend to be covered are ones that can be connected to interesting visuals.
Second, the fact that TV is a visual medium means that the pictures can override the spoken word. Thus, the medium can actually change the message. President Reagan's media people were known for their adept use of this fact. They would put Reagan in photo opportunities where he was doing things that would look good on TV. The TV news would put those images up while they spoke about Reagan and the good visual images would supersede any negative news there was about the President or the country. For example, Reagan was once shown participating in building a house. This was shown while the commentators talked about how there was a lack of affordable housing. The good visual images of Reagan at work offset the bad news and made him look good.
In these ways, the visual medium can affect the message that it carries.