This is a new protection measure to make sure that the story has a good quality poster image as that will show up on Google or your own Magazine.

Step 1:

Select the story on 'My stories' page and click the 'Settings' option on the right side

Step 2: Select the the 'Posters' tab at the top and upload a good quality image for each of the Poster options - portrait, landscape and square. No need to worry about resizing to required sizes - the system does that automatically.