php - How to add Product Image thumbnail to gallery images in Woocommerce?

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I have found a similar question but I can't locate the template which to add the code. I have vanilla Woocommerce using e-Store theme. I've looked through all of the templates and can't seem to locate it.

The original uploaded image is being watermarked but the thumbnail image is left alone. The purpose is to show what the full image looks like without the watermark but at an essentially unusable resolution. I want this thumbnail automatically displayed in the gallery without having to go through and manually add it.

Here is the link to an answer which I believe may help

WooCommerce add product image to gallery automatically (if gallery is empty)