If there is an image with the content of the / blog / whatever you can get the src of the first one with this (same as below).
{% 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:' %}
{% endif %}