We have noticed that brand images are not showing in the Facebook news feed. Instead, a question mark ("?") is showing. If the plugin user doesn't set an image, the default image should be a blue RSS icon. If setting a brand image, it should propagate and show in Facebook.
This problem appears to be related to Facebook. There is one forum post on this plus a bug filed with Facebook. We have contributed to these two threads within the Facebook developer forums.
As soon as we know more about this issue, we will let everyone know.