@saladgoat its always waste of time talking with Facebook developers. And they have proven once again. Anyhow, according to them, adding random images that they deem to be fit is how they have designed the application.
So, the options here are - if you upgrade your Plugin to version 4.0, the images from posts should get picked up and get posted to Facebook as expected, till the time image size is 150px on each side. No need to worry about if it is there in the RSS feed. If that solves your problem, it will be great. Else i will have to use an old API method to publish this content. The issue is, we dont know when the old API method will be phased out. Also i am not too sure about the impact of using old API method on other users.
Let me know.