If you put this tag in your
headinclude template Facebook should use the image you specify when sharing the page.
Code:
<meta property="og:image" content="http://wwwyourdomain.com/image/yourimage.png"/>
The image shouldn't be very big (I'd keep it less than 300x300 pixels if possible, facebook will resize it to thumbnail size.
I don't know about changing it for every article, I suppose it could be done with some custom coding but I don't have the CMS so I can't try.