The image used in the article is determined by the Facebook Open Graph tags set on the page. You can enable and configure Open Graph features in the Social area of the JFBConnect admin section. Click on the Open Graph tab and make sure that Open Graph integration is enabled.
Once done, we'd recommend you enable the setting for automatically using the first image in an article as the image. Next, in the big Defaults box at the bottom, you should put a backup image which is used on non-article pages or articles without an image. Things like a logo are good here. To do so, simply put:
image=http://yoursite.com/logo.jpg
Finally, on any articles or pages where you want to specifically specify a certain image to be used when Liked, you can use the JFBCGraph easy tag, like so:
{JFBCGraph image=http://yoursite.com/best-image-for-article.jpg}
Hope this helps get you all setup and explains things.. but if you have questions, let us know.
Thanks,
Alex