var src = 'randomimage.jpg';

basicLightbox
.create( '<div class="basicLightboxLoader">Loading ...</div>', {
    afterShow: function( instance ) {
        var $element = $( instance.element() ),
            target = $element.find('.basicLightbox__placeholder'),
            image = new Image();

        image.onload = function () {
            target
            .html( image );

            $element.addClass('basicLightbox--img');
        };

        image.onerror = function() {
            instance.close();
        }

        image.src = src;
    }
}).show();