When you share your page with Facebook Like Button or http://www.facebook.com/share.php?u=url, Facebook will automatically parse your Title, Description meta tag and try to find an image to assign it as the preview thumbnail for the sharing.
But sometime the image which you would like to share is not chosen. In order to make sure your favorite image is always correctly populated you should add a tag as below somewhere in your HTML code:
<link rel=”image_src” href=”your_feature_image_url” />
+ image_src: it’s convention of Facebook
+ your_feature_image_url: your full url of the featured image. You don’t need to worry about resizing, Facebook will take care for you.
If you’re using WordPress, I will share with you how I control the featured image which will show on your wall story when you click Like Button on every my post.
First, please this tutorial: http://4rapiddev.com/tips-and-tricks/add-and-get-custom-field-in-wordpress/ to know how to config your featured image by adding and getting a custom field.
Second, for every post which contains some image, you just assign a image url you like best to the value of the custom field named featured_image. For example, I intend to use this image: http://4rapiddev.com/wp-content/uploads/2011/03/Faebook-Custom-Featured-Image-Preview.jpg as the thumbnail image on Facebook as follow:
Before I do the trick, all thumbnail on Facebook for each share on the Wall are not really perfect, all of them are just a Google icon. I love Google but in this case, it’s really not my expectation 🙂
Now it’s better 🙂