Hello There,
Thanks for writing in! To remove the border around the image thumbnails, please add the following css code in the customizer, Appearance > Customize > Custom > CSS
.woocommerce div.product .images .thumbnails>a {
border: none;
}
The thumbnail itself is a cropped version of the original image. The product image dimension can be change in WooCommerce settings. If you want to display the full image, you need to change the thumbnail image dimension. Please check out this topic: https://community.theme.co/forums/topic/product-images-woocommerce/#post-224057
Hope this helps.