Images from tumblr post
I know this is of course not an elegant solution, but it works for now. I used twig filters to extract url and alt tag of the image. 

&lt;div class="blog-post-featured-image"&gt;
            {% if entry.featuredImage|length %}
                {{ snippets.image( entry.featuredImage.first(),entry.url )}}
            {% else %} {# For the tumblr imports, extract first image #}
                {# Extract the src #}
                {% set spliter = entry.articleBody.type('text').first().text|split('src="') %}
                {% set spliter = spliter[1]|split('"') %}
                {% set firstTumblrImage = spliter[0] %}

                {# Extract the alt-Tag of the Featured Image#}
                {% set spliter = entry.articleBody.type('text').first().text|split('alt="') %}
                {% set spliter = spliter[1]|split('"') %}
                {% set imageAltTag = spliter[0] %}
                    &lt;a href="{{ entry.url }}"&gt;&lt;img src="{{ firstTumblrImage }}" alt="{{ imageAltTag }}"&gt;&lt;/a&gt;
            {% endif %}
        &lt;/div&gt;