How to add thumbnail in G Interactive Post

Hello,

I have successfully installed G+ Interactive Posts plugin for WordPress. I can see g+ button on my post. But the problem is that the final interactive post shows full size image (800*529) instead of thumbnail shown in the preview. Therefore, I can neither see the description nor call to action (CTA) button in the final published g+ interactive post?

Check the following links to see 2 screenshots that elaborate the problem:
Correct preview of the interactive post
Final published interactive post in new version of G+. In this, feature image from the post hides post description and CTA.

Qa. How to change the size of the image?

In older version of G+, the interactive post is visible correctly. Here is the screenshot.

Note: I asked the same question in different forum where members suggested to reduce the size of the image in the post itself. But I don’t think this is a practical solution for 2 reasons:

1. G+ interactive post plugin is picking up feature image from the post. If I reduce the size of this feature image, the image will not appear correctly in the twitter card. Twitter image dimensions: 837*438.

2. I have to resize all the feature images.

Qb. What is the simplest solution to fix the bug in new G+?

Qc. Is it possible to edit the image size in the plugin code?

Could anyone help?

Thanks!