AED,
Installing and enabling the Open Graph - Content plugin should help by itself. That plugin will check the article text and should assign the first image it finds in the text as the image to use on a news feed. If there aren't any images in the article text, that can leave it up to Facebook to decide what image to use. In the next release of the Open Graph - Content plugin (in the next few days), there will be more 'fallbacks' if there are no images in the article.. it will then check if there's a category image, and some other areas that may have a relevant image.
As for setting a default image across your site, you can do that very easily, and we definitely recommend it. To do so, go to the Open Graph -> Configuration area of the JFBConnect admin area. In there is a big 'defaults' box. Add the following line (updating the link as appropriate for your site):
image=http://yoursite.com/link-to-your-logo.jpg
That image will then be used if no other images can be found.
Once all setup, you can use Facebook's Debug Tool to test what they 'see' about your pages:
developers.facebook.com/tools/debug
Hope that all helps, but if you have any other questions, just let us know!
Alex