Hello,
In WooCommerce itself, there is no built-in option to set the second product image as the featured image. To set the second product image in WooCommerce, navigate to the product edit page, access the product gallery where you’ve uploaded multiple images, and simply click on the upload image in the gallery.
But if your product has variations, you can add extra images for each variation. Enable the Additional variations images option in Theme Settings > Shop > Variable products: https://take.ms/aRzS2 and upload appropriate images to the variations: https://take.ms/MiDHc
Save the product and check on the frontend if the second image appears when selecting a variation.
Best Regards,