OK. Let me explain something.
In case of Google+ "Link" post our plugin does NOTHING in terms of image or description of the link. Google+ simply does not have ANY way to set or change them. Plugin gives Google+ the LINK and only the LINK and tells it "attach this link to this post message". That's it. Nothing more, nothing less.
Google+ somewhere internally analyzes that link and attaches it to the post message with the title and image it thinks should be there.
There is no way to do anything else. It's totally up to google+ what image to get. In most cases it takes the image from og:image tag. Sometimes it thinks that og:image is not appropriate (bad size, bad quality, guess your own reason) and takes some other image.
There is nothing can be done from our side.