I think we see what the problem is. By default, our Open Graph - K2 plugin will just try to pull the first image that is in the article text out and use that. If it doesn't exist, then no image is added (unless you create an Open Graph Object in JFBConnect for your K2 category). That's all a bit confusing, so let's try the following code fix and let us know how it goes.
Please edit the /plugins/opengraph/k2/k2.php file. Around line 119, you'll see:
$image_type = 'custom_image_first';
Please change that to:
$image_type = 'custom_image_item';
After you make the change, run the URL through Facebook's Debug Tool to see if the image is correct:
developers.facebook.com/tools/debug
If so, then all your pages will update with the proper image over a day or so as Facebook re-scans them automatically.
Hope that helps,
Alex