Marcus,
I don't know about the function to get the Image link for the article image. There probably is a way, but I don't know it off-hand, since we don't have that functionality within JFBConnect. For now though, if you're using the Image setting for each article, and not including any images in the K2 article itself, yes, you'd need to manually add the {JFBCGraph image=xxx} tag to each article.
Good luck,
Alex