[Plugin: Facebook] how to set meta og:image on header?

Hi,

When a FB user left a comment on a post or click on FB like button the post is sent to Facebook, until this ok, but how can I set the plugin use the thumbnail image, and some times it change the crop. So..

How should I set this…

<meta property="og:image" content="url to image" />
<meta property="og:title" content="title of the page" />
<meta property="og:site_name" content="name of the site" />

to use the exactly image size of the thumbnail?

I know is necessary to create a new image with add_image_size function but with which size? 100×100, 300×300 so the thumbnail look correctly in any thumbnail of FB wall or timeline, even in mobile wall..

Thanx for any help ??

https://www.ads-software.com/extend/plugins/facebook/