Facebook thumbnail display

Hi,

So I downloaded and installed “facebook-like-and-send-button” Plugin.
Dead simple to use and worked perfectly right away.
However, a friend of mine pointed out that when he “liked” my blog page there was just a red bar where he expected and normally sees a thumbnail I suppose created by facebook next to his “like” post.

Anyone have nay ideas on how this might be controlled?
Specifically when someone “likes” your post what is the thumbnail facrebook will show to represent your site/blog?

Thanks

Bill