Screenshots are rarely necessary. I’m inclined to say they’re never necessary, but my mother told me that I should never say never so I won’t.
I will say that showing readers images of a site severely dates a book. In my experience, it takes seven to eight months to produce a book through a traditional publishing house. Often, those great screenshots that appear in your January manuscript don’t reflect the site’s interface by September.
Moral of the story: Say no to screenshots.