How do I set the image as a thumbnail of the post on facebook

Hello!
I added the plugin "sociable" in my theme, to share my posts on facebook. The problem is that when I share the post, facebook choose a random image. How do I set the image as a thumbnail of the post on facebook? Where is the code where I have to act? What code should I enter?

thanks

Hi!
Please, contact your plugin developer to fix this.