This will show you how to specify the image you want facebook to use when content from your site gets shared.
you will need to add in the following meta tag
<meta property="og:image" content="http://yoursite.com/image.png"/>facebook will then use the appropriate image.
there are plenty of open graph meta tags to use
see this page on facebook for more
alternatively you can use the following meta tag:
<link rel="image_src" href="http://yoursite.com/image.png"/>