Matthias,
The image can be set using the Open Graph integration features of JFBConnect. You can either let JFBConnect pick the image automatically, for articles, or set it manually using the {JFBCGraph image=http://www.site.com/link-to-image.jpg} tag. For more information on how to use Open Graph, see:
www.sourcecoast.com/jfbconnect/features/...pen-graph-for-joomla
You can turn on the Open Graph integration in the social configuration area of JFBConnect.
Hope that helps,
Alex