Does Facebook keep selecting the wrong thumbnail for WordPress links for you? The solution is to give Facebook some extra instruction about what image to use for the thumbnail, using open graph.
If you use a static frontpage, it’s a simple matter of adding something like:
<meta property="og:image" content="http://samplesite.com/files/2014/05/web-thumb.png" />
to the Full meta tags of your front page.
You can check what facebook will do with your site by using their link debugger: https://developers.facebook.com/tools/debug
WordPress >= 3.9.1 seems to do the right thing for posts according to my testing.