There are some options to cater for how people have their imports set up. The below are all Custom Meta fields, i.e. they end up in wp_postmeta. You may need to name the meta field.

• external_image_url – you can import the featured image and the image galleries here. The first image is the featured image, subsequent images (separated by | pipe) are treated as gallery images
• gallery_urls – you can import your gallery image URLs here if you wish, instead of putting everything in the external_image_url field

That’s all you need to know really – if you’re dealing with variations, the same applies – load the variation custom post meta field of external_image_url and optionally gallery_urls with | pipe separated image URLs.