The ability to post to a stream is through the Social Toolbar 'Create Post' button. When you use this, you're actually just posting the URL to the page that you're on. The message that you can enter is just a comment to go along with the page URL. Then, when the post shows in Facebook (or other social network), any images or further descriptions are scraped from the Open Graph tags on the page.
So while you won't be able to specify an image on that 'Create Post' dialog, you can explicitly set the Open Graph image on the specific page you are sharing. JFBConnect attempts to determine the best image by article and category content, but you can set an image by just inserting the SCOpenGraph easy tag in your article content.