cannot post right image of k2 item

Dimitris,
You don't look to have JFBConnect setup properly. There are a few issues I can see that need to be taken care of before you can work with the Open Graph stuff:
* The JFBCSystem plugin doesn't look to be enabled. That's required for almost all JFBConnect functionality
* You have other extensions on your site that are including the Facebook Javascript library. One is your Fan Box / Like Box module and the other is the Like button on that page. You'll need to either disable those extensions or at least have them disable their inclusion of the Facebook Javascript library. JFBConnect has replacement options for both of those widgets.

Once that's taken care of, please install the Open Graph - K2 plugin, and that should automatically insert the proper og:image into your page. Right now, there's no Open Graph image tag being set, so Facebook is guessing at the proper image to use.

Hope that helps,
Alex