For everything Facebook does right they don't pick images very well.
If you have Facebook Connect setup you can add an image for your "app" in the developer's page for the app and that image should be used.
If you don't have Facebook Connect enabled/setup then you can manually add this line line to your
headinclude template.
HTML Code:
<meta property="og:image" content="http://www.youursite.com/images/myimage.jpg" />
Where of course you replace the URL with your own URL to an image.
Unfortunately this image won't vary by thread, it will have to be the same image for all your likes- I personally use a forum logo.
Note- changes will not be immediate- facebook stores a cache of your page so it may take several days for changes to appear on a page that has already been "liked." Try it on a brand new page/thread to make sure it is working.