How, using robots.txt, can I specify which image Facebook uses as the thumbnail when the link to a specific site is posted either on someone's Facebook wall or via email?
Currently, for a particular site, there are 8 images available to choose from but none are representative of the sites' content are give a clear image of what's available on the site. So, I've created a thumbnail size screenshot of the site and would like to point the facebook bot (facebookexternalhit/1.0) to that specific image.
Note this won't necessarily change the images displayed on existing wall posts and FB emails as the FB bot may not have visited your page, and thus updated the cached image, since you made the changes. However, it should ensure your_chosen_image is used in future posts.