Hey Jacquie,
From what I know, currently there isn't a way to specify an image for opengraph when sharing a post. There aren't any plugins that can achieve this.
The way this system works is that it will first detect if you have a cover image for the post and use that image for opengraph. If you there isn't any cover image, the system will use the first image in your post in opengraph.
For example, you can see that in the attach image, for that post og uses the first image because there is no cover image present in the post.
Hope this clarifies your question.