how to change the main photo on a movie page

Hi, my main picture: https://www.imdb.com/title/tt28697858/?ref_=ttmi_tt on our film page is of 2 people walking (that's next to the description on the main picture). I need to keep that photo, but just have it as a still frame. I need to have this still frame as the main picture instead: https://www.imdb.com/title/tt28697858/mediaviewer/rm109988353/  of the girl in blue crying on the guy's shoulder. (I want to still keep both pictures - not delete - but exchange them where the other becomes the main one showing). Thank you.