This page was printed on Jan 24, 2019. For the current version, visit https://help.shopify.com/en/themes/customization/products/features/add-captions-or-labels-to-product-images.
Product images don't have metadata that could be used as captions in your Liquid templates. You can work around this by using the product image filename to come up with a descriptive caption using Liquid filters.
Adding an image caption using Liquid
When each product image illustrates a variant (and only one image does), and when your product images are dragged and dropped in the same order as your variants are listed, you can use this Liquid for your image caption: