Image shared through share button

Hi, I have a problem with the image shared by the share button. The image is one of those in the right sidebar and not in that of the article to share. The one visualized by the Like button is the correct one. By using the open grapgh debugger the following error appear:

The ‘og:image’ property should be explicitly provided, even if a value can be inferred from other tags.
Provided og:image is not big enough. Please use an image that’s at least 200×200 px. Image ‘http://www.militant-blog.org/wp-content/uploads/2013/08/foto.jpg’ will be used instead.

Moreover, for the Like button the following warning appears:

og:image is missing. The og:image meta tag is necessary for Facebook to render a News Feed story that generates a high click-through rate.

This is one of the pages in which the problem happens
http://www.militant-blog.org/?p=10164

How do I solve the problem? Is there some particular setting in the options? All the articles have the featured image correctly set

http://wordpress.org/plugins/facebook/