Hi,
It already works like this by default. You don't need to set the images for each variants. The system will automatically use the images of the main product.
On your website, this doesn't work because of something which hides the variant images for some reason.
It's likely to come from your Joomla template or a lazy loading image extension.
A simple way to circumvent that would be to add such CSS on your website:
.hikashop_product_page img {
opacity: 100 !important;
}
That should override what that extension/template does. Otherwise, you'll have to try with the default template of Joomla and see if you still have the same problem. If not, contact your template provider for help.