Edwin,
Facebook uses Open Graph tags to set the title, description and image to use when sharing your content. JFBConnect will try to set these automatically from what it can detect on the page. If you haven't already, please enable the Open Graph plugins for extensions you have installed and are using, like Open Graph - Content. Those plugins help JFBConnect to detect the best image to use based on an article's content, for example.
Beyond that, you can also do any of the following to help set the images for your page:
* In the JFBConnect -> Open Graph -> Configuration -> Defaults area, set a tag like the below to set a 'default' image that should be included on all pages as an option:
image=http://yoursite.com/link-to-image.jpg
* On any page, add the following tag to set it as the highest-priority image, which overrides any other images that may be set by the plugins or default tag:
{SCOpenGraph image=http:/yoursite.com/link-to-image.jpg}
I hope that helps get you going,
Alex