Lists: Allow a list creator to select one of the films to act as a thumbnail graphic

On a lot of my lists the early ones can tend to be without a graphic, leaving the list with a blank thumbnail, which can't really help them stand out in a list (and a well chosen thumbnail can help summarise the contents of the list which is especially useful on a film's main entry where they don't have the description). So on a lot of my lists I have taken to adding in a film again and moving it to the top of the list:http://www.imdb.com/list/QUzo9z00B8E/... the sidebar here for an example involving that second list:http://www.imdb.com/title/tt1733677/This isn't ideal, as it means when the list is sorted, the film appears twice next to each other.