Poster Frame for Social Media Share

Hi SI Team

I want to share an article on Facebook.
This is the article in question.
http://finalbug.net/blog/entry/397-v-o-d-training-for-fcpx-in-german
When I do so I am offered a choice of 3 Thumbnails!

None of which have anything to do with the main image for the article that I am trying to publish. Am I missing something?!?

thanks

Paul