How to add primary image posters in TV episodes?

Why only stills can be made as primary image in episodes (unless you upload a poster first)? Wider stills are displayed in episode list of series, what criteria is used on selection of a particular still? I have to upload a poster a "still frame" and make that a primary image at https://www.imdb.com/title/tt21289534/. Can someone please change the type (https://www.imdb.com/title/tt21289534/mediaviewer/rm2887608321) from still to "poster" (without changing primary)?