The generally expected user experience on a product page is to select the variant using a dropdown selector or swatches. As multiple product images may be associated with a single variant, attempting to select the variant through selecting a thumbnail image becomes a confusing process. Additionally, Shopify products can contain up to 3 options full of various variants; these combination selections can be particularly problematic, as each combination would require a unique product image. To echo Shopify’s warning about this user experience:
“Most themes update the main image when a variant with an image is selected. The update never happens in both directions though, because having a change of the main image automatically update a variant selection might confuse shoppers into adding the wrong variant to the cart. So, proceed with caution in applying this customization to your store.”
While this functionality is not built-in to the theme, it may be possible for a custom developer to create this feature, following your specifications - for which we suggest contacting one of our recommended experts.