You can set the primary image for a Product using one of your Variant images in your App.
Go to the Products section of your App and select the product which contains the variant whose primary image you want to change.
Tap the variant that contains the image, then tap the variant to view it in full screen.
Tap the image to enter the image viewer, and then touch the “…” button at the top right hand corner of your screen.
Click "Set as primary image" to select the image from that variant as the primary image. This will be the image used in the variant list views for that product.