replace the small images with dots

Hello,

You can do using custom CSS. Please add this CSS in your site and check

@media only screen and (max-width: 600px) {
    li.product_thumbnail_item.nslick-slide {
        text-align: center;
    }
    li.product_thumbnail_item.nslick-slide:after {
        content: "⚫";
    }
    img.attachment-woocommerce_gallery_thumbnail.size-woocommerce_gallery_thumbnail {
        display: none !important;
    }
}