The Algolia Shopify app only supports the primary image and a variant image by default. We use the variant image if it exists, and if not the primary image is used:
If you'd like to add additional images, they'll need to be added as metafields. If those images need to be displayed on the front-end in some manner, you'll need to customize the InstantSearch implementation to use the information from those metafields.
Information on customizing InstantSearch within Shopify is available here.