"Suggest as primary image" option is allowed in the same menu but when editing posters. So you need to add an image as a poster or edit an existing poster, and then check "Suggest as primary image".
Hi, Kevin Hicks. For episode-type titles, the width-to-height ratio has to be greater than 1:1, or preferably equivalent to or near 16:9, and the category/type of the image has to be "Still", whereas for non-episode-type titles, the width-to-height ratio has to be less than 1:1, or preferably equivalent to or near 2:3, and the category/type of the image has to be "Poster". With those conditions met, the option "Suggest as primary image" should appear in the menu reached by clicking the "Edit Tags" button/hyperlink.
Jeorj Euler