blog/_src/_includes/picture.html

{% case include.size %}

{% when 'phototeaser' %}
     {% assign pictureSize = 'w_940,h_700,c_fill,q_auto:good,f_auto' %}
     {% assign pictureSizeRetina = 'w_1880,h_1400,c_fill,q_auto:good,f_auto' %}
{% when 'photothumb' %}
    {% assign pictureSize = 'w_190,h_190,c_thumb,q_auto:good,f_auto' %}
    {% assign pictureSizeRetina = 'w_380,h_380,c_thumb,q_auto:good,f_auto' %}
{% when 'featured' %}
    {% assign pictureSize = 'w_275,h_100,c_fill,q_auto:good,f_auto' %}
    {% assign pictureSizeRetina = 'w_550,h_200,c_fill,q_auto:good,f_auto' %}
{% else %}
     {% assign pictureSize = 'w_940,q_auto:best,f_auto' %}
     {% assign pictureSizeRetina = 'w_1880,q_auto:best,f_auto' %}
{% endcase %}

{% if include.file %}
    {% assign pictureSrc = include.file %}
{% else %}
    {% if page.image %}
        {% assign pictureSrc = page.image %}
    {% else %}
        {% assign pictureSrc = post.image %}
    {% endif %}
{% endif %}

<img src="{{ site.media_url }}/{{ pictureSize }}/media/{{ pictureSrc }}" srcset="{{ site.media_url }}/{{ pictureSize }}/media/{{ pictureSrc }} 1x, {{ site.media_url }}/{{ pictureSizeRetina }}/media/{{ pictureSrc }} 2x" alt="{{ post.title }}">