This can be done using [{PRODUCT_IMAGE}] tags: [{PRODUCT_IMAGE1}], [{PRODUCT_IMAGE2}] etc.

It is recommended to add pictures that will be used in the HTML Template to the main product of the variation group to avoid issues with numbering used in the PRODUCT_IMAGE tags.

Note: Product Image needs to be inserted in HTML Template as a HTML tag. For example: <img src="[{PRODUCT_IMAGE1}]" />.