If you go into the Open Graph -> Configuration area of JFBConnect, there is a big "Defaults" box. In there, add a line like:
image=http://yoursite.com/link-to-image.jpg
That will set a default image to be used on all pages of your site. If a 'better' image is automatically selected by JFBConnect, or if you add the {SCOpenGraph image=http://yoursite.com/better-image.jpg} tag to the page, that will be used first instead of the default set above.
Once set, it can take Facebook a few days to see the new Open Graph image tag. You can also use the
Facebook URL Debug Tool
to test pages of your site to make sure the new tag is being properly set.
I hope that helps, but if you have any other questions, just let me know.
Thanks,
Alex