Hi,
I’m sorry to say this won’t work with your site out of the box, because this plugin pulls the wordpress article thumbnail and nothing else.
The plugin does all that can be done to make it work.
I haven’t heard of one single case where it didn’t work, so i’m pretty sure you missed out one installation step, e. g. rebuilding thumbnails of your existing pictures. This is vital for the plugin to work even with your “old” posts.
Please read the complete manual with additional hints here, i’m sure it works for you:
http://www.mindgears.de/2011/02/wordpress-plugin-add-facebook-share-thumbnail-meta/
