If you use WPBook to push your WordPress posts to Facebook, this is for you.
I followed these instructions and it worked in no time.
But just to be brief, here is what I did:
I added the following text to the functions.php file of my WordPress theme:
add_theme_support( ‘post-thumbnails’ );
set_post_thumbnail_size( 50, 50 ); // 50 pixels wide by 50 pixels tall, box resize mode
Then, whenever I upload an image, there is a little button next to the “Insert Post” button that is labelled “Use as featured image”. I click that on the image I want to show as the thumbnail in Facebook.
It would be nice if it just automatically selected the first image in the post as the Featured Image, but it doesn’t. I have to do it manually. If I forget, it uses an image from the sidebar of my post. :-(