Hmm, one problem is that the "Like" button doesn't allow you to select an image, and FB tends to pick a wrong one (say, a button on the sidebar, instead of an image in the actual post).
I googled, and this link seems to explain the problem:
Basically you need to add a meta tag to the page:
<meta property="og:image" content="(the image's URL)" />
Is it possible for this plugin to, in the future, add that tag? Maybe pick the first linked image in the post, or, even better, just add a text box for the user to paste an URL?