Twitter Card Preview is failing with twitter:image
I made a test with this values

&lt;meta name="twitter:url" content="http://liberalgeek.com/content/why-do-atheists-celebrate-christmas" /&gt;
&lt;meta name="twitter:card" content="summary" /&gt;
&lt;meta name="twitter:creator" content="@kennywyland" /&gt;
&lt;meta property="twitter:description" content="I can&amp;#039;t speak for all atheists, I can only speak for myself, so please leave a comment below and explain why you celebrate Christmas! When I hear someone ask, &amp;quot;Why do atheists celebrate Christmas?&amp;quot; my first thought often is: Better Question: Why do Christians celebrate Christmas?" /&gt;

&lt;meta name="twitter:title" content="Why do Atheists celebrate Christmas?" /&gt;

&lt;!-- the following line causes the problem --&gt;
&lt;meta name="twitter:image" content="http://graphics8.nytimes.com/images/2012/02/19/us/19whitney-span/19whitney-span-articleLarge.jpg"&gt;


Changing the image, the card works normally in the preview tool.
I would try to check if you have some restriction on the file.
Have a look at the crawling section of the API docs  


  Twitter's crawler will respect robots.txt when scanning URLs. If a
  page with card markup is blocked, no card will be shown. If an image
  URL is blocked, no thumbnail or photo will be shown.


UPDATE
As suggested by the author of the question, changing robot.txt could not be enough. Since Twitter could have cached your file, you need to wait at least 24 hours to let it expires.
I add that the same problem could be found not just on robot.txt file, but also on images. Indeed, wrong Twitter cache could lead to a 404 error and a broken preview image.