I have enabled the social features of All in One SEO and now the warnings disappear.
Just a small note, I replaced the default image with the following
That is clearly bigger than 200x200 but the API of facebook still have the warning that it is not correct.
In order to make that warning disappear i added the URL of the image in the "Use Custom Field For Image". However I am not sure that I really want to do this because in the case of a new post that contain an image this warning actually works as a feature:
Provided og:image is not big enough. Please use an image that's at least 200x200 px. Image 'http://www.francescolelli.info/imgIndexArticle/social_network.jpg' will be used instead.
In other words Publicize replace the default image with the one contained in the post.
Is there any other way to trigger this behavior?