There's a few ways. If there is an Open Graph plugin for the component you're using (like Joomla content, K2 content, JomSocial, etc), then enable that and it should insert it's best guess for the og:image into the page.
Alternatively, on each page, you can add the following tag to specify the image for that page:
Finally, in the Open Graph configuration area of JFBConnect, you can add a 'default' image to be used on all of your pages in case another isn't specified. That's great for a logo or something similar. To do so, add the following into the Defaults box:
Hope that helps,