Unfortunately, that's how Facebook works. The Open Graph specification states that the order of the images in the HTML is how they should be used. JFBConnect always sets the default image as the last og:image in the page so it should be the 'least' priority. However, Facebook will generally look at all images that are specified and pick the largest image, based on its dimensions.
There's not a way, with Facebook, to specify a priority. All I can recommend is that your default image has a good size, but isn't huge, and all images within your content should be even larger so that Facebook chooses those instead.
I hope that helps explain, but if you need anything else, please let us know.
Thanks,
Alex