After reading Craig Shemin's interview and seeing that they didn't want to use screen grabs if they didn't have to, I kept thinking about how every page has at least one screen grab (even for characters who should be easy to find three actual photos of), and kept wondering if he meant they wanted each page to include at least one photo that wasn't a screen grab or wanted the main picture to be a non-screen grab. But then I thought of something else: Many screen grabs are accompanied by information about the scenes in question, so perhaps that's why they included them in photos of main characters like Gonzo*, Animal*, Rowlf, and Scooter, because they wanted to include those moments.
*though I'm not really sure whether the images in question are screen grabs or photos taken for the storybooks/promotional purposes. Though the one on the Animal page seems to have a "faded" quality that most of the screen grabs have.