Matrix gallery with Imager plugin
Assuming it's the field portfolioImage that can have multiple assets, try this:

{% for block in entry.portfolio %}
    {% switch block.type %}

    {% case "images" %}

    {% for image in block.portfolioImage %}
        {% set transformedImages = craft.imager.transformImage(image,
            [
                { width: 1200, ratio: 4/3 },
                { width: 992, ratio: 4/3 },
                { width: 768, ratio: 4/3, jpegQuality: 60 },
            ],
            {
                format: 'jpg',
                allowUpscale: false,
                mode: 'crop',
                jpegQuality: 80,
                interlace: true
            }
        ) %}
        &lt;img class="img-responsive"
             src="{{ transformedImages[1].url }}"
             sizes="50vw"
             srcset="{{ craft.imager.srcset(transformedImages) }}"
             alt="{{ image.title }}"&gt;
    {% endfor %}

    {% endswitch %}
{% endfor %}