Plugin to embed WordPress Blog into Facebook Platform. Requires PHP 5. Requires ability to access your WordPress blog via HTTPS (SSL).
In the wpbook/theme directory, there is an index.php file. Most of what you want is there.
There's also a default/style.css which basically mimics Facebook's styles, as well as some other files for processing comments and the like.
Actually, what's happening is that Facebook is choosing an image at random for your posts.
This happens because either you have not set a "featured image" for the post, or your theme doesn't support "featured images."
If you haven't set a featured image, do set one - WPBook will pass that to Facebook along with the post.
If your theme doesn't support featured images, you'll need to add code to your theme's functions.php file.