Great App, poor support.
For the "common variant image" set-up, here is what I find, through a really hard time.
1. Common variant image cannot be set as the variant key image. If set, there will be no common image.
2. Rule 3(Common variant image set-up) and Rule 4(Featured image set-up) is conflicting with each other. Checking Rule 4 will make common variant image disappear.
I was struggling on the "common variant image" set-up. It was really a hard time communicating with their support, who did not appear to understand my problem nor the app itself. At the end, I figured out the setting I needed by myself.