In nopCommerce 3.1 you can map attributes to pictures, which is great by the way.
But the image that is set to the default picture is not the full size image and when you enable the default image click to zoom it displays the small image rather than the full image.
So maybe the ProductVariantAttributeModel should have 2 properties - PictureUrl (which it already has) and FullPictureUrl.
This way you can set the PictureUrl to the main-product-img- and the FullPictureUrl to the main-product-img-lightbox-anchor-.
Hope this helps!