Create a Featured Image

Ok so I have searched around and come up with this information
https://stackoverflow.com/questions/33618706/embedly-not-using-featured-images
https://docs.embed.ly/docs/cards#customize
So the end game is to get a image created in the post or better still a featured image created . The reason is when the post is parsed to social media a lot require a featured image
I know what I want to do I just dont know how to do it!