From http://ecommerce.shopify.com/c/ecommerce-design/t/reside-thumb-image-for-blog-couture-theme-148530

new_gist_file

{% assign src = false %} 
{% if article.content contains "<img" %}

    {% assign src = article.content | split: 'src="' %}
    {% assign src = src[1] | split: '"' | first | replace: '//cdn', 'http://cdn';; | replace: 'http:http://';;, 'http://';; | remove: 'https:' %}

    {% comment %}
        Let's assume you always add images to the blog as large.
    {% endcomment %}

    {% if src %}
        {% assign src = src | replace:'_large','_icon' %}
    {% endif %}

    <img src="{{ src }}" alt="article.title" />

{% endif %}