The New York Times has far more multimedia on their website than that of, for example, St. Paul's Pioneer Press.
On almost any given day, the Times will have a slideshow featured on their homepage, as well as pictures accompanying major stories and videos.
The Press will usually have maybe one or two pictures featured, but I have never seen a slideshow or any videos on the homepage.
I feel vidoes and pictures can complement a news story in that it attracts the reader, and pulls people to read that story first.
As far as slideshow's, they complement news because the pictures help to tell the story.
The Times will have copy with their slideshows whenever they are up, which tells you what is going on in the pictures and how it realtes to the stories. When they just have a series of pictures, they'll generally just have one tagline for all of them, but that is usually for an event that just took place that most people already know what is going on.
The writing that goes along with the multimedia is always very short and to the point. They want the pictures or videos to be the focus of the reader's attention.