I am 99% certain that this used to work when my domain got approved for twitter cards many months ago (http://alc.cx). Right now, twitter does not display the photo card for tweets that contain links such as this one: http://alc.cx/1/p/g .
Example tweet: https://twitter.com/meilleurerep/status/449579456093773824
I checked this again with the validator which indeed does not display the picture but says everything is fine, especially for the twitter:image:src meta tag. The top left bullet is red though but I have no idea what this means. Screenshot of the validator below:
I tried to add a .png extension to the picture tag, added a robots.txt file (there was none) but I could not see any change in the output of the validator.
Help of any kind would be most appreciated !