In the JSON file images may be stored as an array of objects that consist of e.g. image id and image URL.
Image URLs are stored as a property of the object in the product_images array
In this case use the following settings:
Stockeo settings when Image URLs are stored as a property of the object in the product_images array
Image URL: product_images[0].image
Image URL: product_images[1].image
Image URL: product_images[2].image
Please note that the indexing of array elements in JSON starts at 0.
You can add fields up to the highest expected number of images. This will not cause an error, even if there is no link to the image on the given index.