If wordpress featured image is not showing on facebook, twitter when you try to share your blog post link on those social media platforms, or it defaults to using the header image, try and install the OPEN GRAPH wordpress plugin via :
https://wordpress.org/plugins/opengraph
There is nothing to configure and there is no admin page. Just install and try sharing your posts again and Facebook, Twitter will display the right wordpress post image.
By default, it will use whatever standard WordPress data it can to populate the Open Graph data.
The Open Graph protocol enables any web page to become a rich object in a social graph. Most notably, this allows for these pages to be used with Facebook’s Like Button and Graph API as well as within Twitter posts.
The Open Graph plugin inserts the Open Graph metadata into WordPress posts and pages, and provides a simple extension mechanism for other plugins and themes to override this data, or to provide additional Open Graph data.
This plugin does not directly add social plugins like the Facebook Like Button to your pages (though they’re pretty simple to add). It will however make your pages look great when shared using those kinds of tools.
If there are other plugins that can fix this problem, feel free to share.