atom.xml

{% assign cover_image = post.path | prepend: '/' | prepend: site.source %}
{% capture cover_image_exists %}{% cover_exists {{ cover_image }} %}{% endcapture %}

{% if post.image and cover_image_exists == "true" %}
    <media:thumbnail xmlns:media="http://search.yahoo.com/mrss/" url="{{ site.url }}{{ post.url }}{{ post.image }}" />
{% else %}
    <media:thumbnail xmlns:media="http://search.yahoo.com/mrss/" url="{{ site.url }}/images/social/headshot.jpg" />
{% endif %}