Champions | League of Legends

Kind of just a random question, but on the champion page, I notice only a few champs have a gif icon and if you click on them, it will play a short video with a picture of them and some sound. Why do these champs have this and not others? The ones I see are, Zed, Sona, Garen, and Vi. those are the ones I can recall seeing it with and i'm just curious about them if it switches or something. just random curiousity.