I thought i take some time to help people who could have the same issue.

My problem was that the K2 articles links shared in Facebook did not display the thumbnail related to the Article image.
1- For people using Artisteer to make templates check this first:
www.artisteer.com/?post_id=133141&p=forum_post&forum_id=20
2- Interesting Link is also this one:
community.getk2.org/forum/topics/item-image-as-facebook-sharing

3- My Problem was described in Link above but i had to read it again to find the problem...
The Height and the Width from the Picture to use as thumbnail MUST be more than 50px!

i did set the K2 settings to 50px for XS width... so the portrait formated pictures did show up and landscape did not because the height was less than 50 px!

It was a nightmare asking me why some items did show up and some others not.