
The current rule used is - highest res poster to main.
Some shows have promotional posters as main, with the date of the new release (e.g. NEW SERIES Streaming Jan 15), some shows have posters with the schedule as main (e.g. SUN 10|9c), some posters with the network logo as main (e.g. CBS) and some "clean posters" as main (poster containing only the main art and show name).
Promotional posters are good for using before they start/re-start to air/stream, for quickly get the date they'll be available without opening the actual show page... but using them afterwards... debatable.
Also, from the last 3 categories, should we stick with just one?
... and uploading crappy tvdb posters should be banned. (low res 680x1000px/ alien aspect ratio / blurry / highly compressed / bad cropped)